diff --git a/docs/ADR-027-rich-search-filters.md b/docs/ADR-027-rich-search-filters.md
index 7b1a7924..d3721f37 100644
--- a/docs/ADR-027-rich-search-filters.md
+++ b/docs/ADR-027-rich-search-filters.md
@@ -1,6 +1,6 @@
 # ADR-027: Rich Search Filters for Semantic Search
 
-**Status**: Proposed
+**Status**: Accepted (Phase 1 implemented; Phases 2–3 deferred)
 **Date**: 2026-06-02
 **Depends On**: ADR-012 (Unified Multi-Algorithm Search), ADR-014 (BM25 Search), ADR-019 (Verify-on-Read for Semantic Search)
 **Tracking**: Astrolabe Cloud POC Deck card #177
@@ -66,16 +66,33 @@ Filters can only be applied to fields that exist in the Qdrant payload (built in
 
 | Desired filter | Payload field | Type | Status |
 |---|---|---|---|
-| Modified-date range | `modified_at` | `int` (Unix ts) | ✅ **Ready** — numeric, range-filterable today |
+| Modified-date range | `modified_at` | `int` (Unix ts) | ✅ **Ready** — value present on every point; needs a payload index (added in Phase 1, no content re-index) |
 | Document type | `doc_type` | keyword-indexed `str` | ✅ Implemented |
 | Directory / path | `file_path` (files only) | `str` | ⚠️ Stored but **not keyword-indexed** — prefix/match needs a payload index |
 | Tags | — | — | ❌ **Not indexed** — no `tags` field is written during scanning |
 | Category (notes) | — | — | ❌ Not in payload — fetched from the Notes API at verify time only |
 
+`modified_at` is a deliberate cross-app normalization the scanner already performs: the Notes API
+and Deck return int Unix seconds natively, News timestamps are unit-normalized to seconds, and
+WebDAV's RFC-1123 `getlastmodified` *string* is parsed to `int(dt.timestamp())`
+(`client/webdav.py`). One `int` field therefore covers every doc type.
+
 Two consequences:
 
-- **`modified_at` is the cheap win.** It is already a numeric Unix timestamp on every point, so a
-  `Range` condition works against the existing index with no re-index.
+- **`modified_at` is the cheap win — but it still needs a payload *index*.** The value is on every
+  point, so **no content re-index** is required. However, Qdrant only evaluates a `Range`
+  efficiently against an *indexed* field; without an index every dated query full-scans the
+  collection (and 400s on Qdrant Cloud strict payload-validation mode). Phase 1 therefore adds a
+  single `modified_at: INTEGER` entry to `_PAYLOAD_INDEX_FIELDS`
+  (`vector/qdrant_client.py`); the existing idempotent `_ensure_payload_indexes()` startup path
+  creates it on new **and** existing collections with no operator action. `INTEGER` (not `FLOAT`)
+  because the stored value is an int Unix-second timestamp.
+- **Why not a Qdrant `datetime` index?** Qdrant's `datetime` index / `DatetimeRange` would let us
+  filter with RFC 3339 strings directly, but its indexer only ingests *string* payload values
+  (`value.as_str()` in Qdrant's `numeric_index/value_indexer.rs`) — it silently skips integer
+  payloads. Adopting it would mean re-storing `modified_at` as RFC 3339 strings on every point,
+  i.e. the full re-index Phase 1 is designed to avoid. We instead keep int storage + a numeric
+  `Range` and accept RFC 3339 only at the request boundary (see §3).
 - **Tags / path / category are not free.** `file_path` filtering needs a Qdrant payload index
   before `MatchText`/prefix matching is performant; `tags` and `category` are not in the payload at
   all and require extending `processor.py` plus a full re-index. Conflating these with the date
@@ -83,40 +100,97 @@ Two consequences:
 
 ## Decision
 
-### 1. Generalise the filter contract
+### 1. Generalise the filter contract through one shared helper
 
-Every structured filter follows the `doc_type` path: **tool parameter → `search()` keyword arg →
-`FieldCondition` appended to `filter_conditions` → `Filter(must=[...])` on both prefetch branches.**
-Filters are always applied at the Qdrant layer, **before** verify-on-read (ADR-019), so that
-`verified_chunk_count` / `dropped_document_count` describe the already-filtered set and the verifier
-never wastes Nextcloud round-trips on documents the filter excluded.
+Every structured filter follows the `doc_type` path: **tool parameter → explicit `search()`
+keyword arg → `FieldCondition` in the shared `filter_conditions` builder → `Filter(must=[...])`
+on both prefetch branches.** Filters are always applied at the Qdrant layer, **before**
+verify-on-read (ADR-019), so that `verified_chunk_count` / `dropped_document_count` describe the
+already-filtered set and the verifier never wastes Nextcloud round-trips on documents the filter
+excluded.
 
-Date/range bounds use `qdrant_client.models.Range`:
+**The filter is added to the `SearchAlgorithm` ABC contract, not just one algorithm.** The two
+algorithms that back the search surfaces — `BM25HybridSearchAlgorithm` (the MCP tool path,
+`nc_semantic_search`) and `SemanticSearchAlgorithm` (the dense-only visualization / `/api/v1`
+path, `api/visualization.py` + `auth/viz_routes.py`) — today build a *byte-identical*
+placeholder + ownership + `doc_type` filter block. Rather than copy the new condition into both,
+the common block moves into one helper, `search/access_filter.py::build_base_filter_conditions`,
+and both algorithms call it. New filters are therefore honoured on **every** path (hybrid and
+dense-only) by editing one function:
 
 ```python
-from qdrant_client.models import FieldCondition, Range
-
-if modified_after is not None or modified_before is not None:
-    filter_conditions.append(
-        FieldCondition(
-            key="modified_at",
-            range=Range(gte=modified_after, lte=modified_before),  # None bounds are open-ended
+# search/access_filter.py — the single source of the filter contract
+def build_base_filter_conditions(
+    user_id, accessible_owners=None, doc_type=None,
+    modified_after=None, modified_before=None,
+) -> list[Condition]:
+    conditions = [get_placeholder_filter(), build_ownership_filter(user_id, accessible_owners)]
+    if doc_type:
+        conditions.append(FieldCondition(key="doc_type", match=MatchValue(value=doc_type)))
+    if modified_after is not None or modified_before is not None:
+        conditions.append(
+            FieldCondition(
+                key="modified_at",
+                range=Range(gte=modified_after, lte=modified_before),  # None bounds are open-ended
+            )
         )
-    )
+    return conditions
 ```
 
 `Range` treats `None` bounds as open, so the same condition serves after-only, before-only, and
-both-bounds queries. Validation that `modified_after <= modified_before` lives in the Pydantic
-request model, not the algorithm.
+both-bounds queries. New range/match filters are added here once; each algorithm wraps the
+returned list in `Filter(must=...)` and may append its own additive conditions afterward (e.g.
+`SemanticSearchAlgorithm`'s opt-in ACL pre-filter, which `BM25HybridSearchAlgorithm` does **not**
+apply).
+
+`modified_after` / `modified_before` are promoted to **explicit named keyword params** on
+`SearchAlgorithm.search()` (and both concrete impls), exactly as `accessible_owners` was — not
+left in `**kwargs`. Explicit params keep them discoverable and make a misspelled keyword a type
+error instead of a silently-ignored filter. When `doc_types` is a list, the tool's per-type
+dispatch loop forwards the same `modified_after`/`modified_before` to each per-type
+`search()` call.
+
+**Input validation.** FastMCP exposes no request-model object to hang a Pydantic
+`@model_validator` on, so validation is split across three layers, each handling what it can
+express:
+
+- **Per-argument scalar bounds** use `Annotated[..., Field(...)]` on the tool signature — FastMCP
+  builds the input schema from these and rejects bad values before the body runs. This is the
+  repo's first use of `Annotated`/`Field` on a tool; the existing numeric knobs (`limit` `ge=1`,
+  `score_threshold` `0.0–1.0`, `context_chars` `ge=0`) are tightened the same way and the pattern
+  is what future filters reuse.
+- **Format normalization for the date bounds.** `modified_after` / `modified_before` are typed
+  `str | int | None` and accept an **RFC 3339 / ISO 8601 datetime** (e.g. `"2026-01-01T00:00:00Z"`,
+  naive ⇒ UTC) *or* Unix seconds. A shared `utils/validation.py::parse_modified_timestamp` helper
+  normalizes either to int Unix seconds (the payload representation) and raises `ValueError` on an
+  unparseable value; the tool converts that to `McpError`, the HTTP endpoints to a 400. The same
+  helper is reused by `nc_semantic_search`, the `/api/v1` search endpoints, and the viz route so
+  every surface accepts identical formats.
+- **The cross-field invariant `modified_after <= modified_before`** can't be a per-field
+  constraint, so it is an explicit guard (on the *parsed* int values) that raises
+  `McpError(ErrorData(code=-1, ...))` in the tool — the established input-error idiom (mirrors the
+  `VECTOR_SYNC_ENABLED` guard) — and a 400 on the HTTP endpoints.
 
 ### 2. Phase the rollout by payload readiness
 
 - **Phase 1 — modified-date range (this ADR's committed scope).** Add `modified_after` /
-  `modified_before` (Unix seconds, UTC) to `nc_semantic_search` and `bm25_hybrid.search()`. No
-  re-index. Ship the frontend chip UX against this plus the existing doc-type filter to prove the
-  end-to-end plumbing on fields that already exist.
+  `modified_before` (RFC 3339 / ISO 8601 at the boundary, Unix seconds accepted too) to the
+  `SearchAlgorithm` contract and both algorithm impls (so the MCP tool *and* the dense-only
+  `/api/v1` path honour it), plus a `modified_at` INTEGER payload index. No content re-index. Ship
+  the frontend chip UX against this plus the existing doc-type filter to prove the end-to-end
+  plumbing on fields that already exist.
+  - **`nc_semantic_search_answer` is explicitly deferred, not part of Phase 1.** It is a thin RAG
+    wrapper that today threads only `query`/`limit`/`score_threshold`/`fusion`/context options to
+    `nc_semantic_search` and does not even expose `doc_types` — it always searches cross-app. Date
+    scoping is a search-box affordance, not an answer-synthesis one, and there is no UI surface for
+    it on the answer path. When demand appears it can be threaded through using exactly the same
+    parameter + `parse_modified_timestamp` pattern; doing it now would add an unused parameter and
+    widen the change for no user-visible gain.
 - **Phase 2 — directory / path.** Create a Qdrant payload index on `file_path`, add a `path_prefix`
-  parameter, and add an `NcFilePicker` folder chooser. Scoped to `doc_type == "file"`.
+  parameter, and add an `NcFilePicker` folder chooser. Scoped to `doc_type == "file"`: the frontend
+  must **hide/disable the path filter unless the file doc type is selected**, because `file_path`
+  is only written to the payload for `doc_type == "file"` (`processor.py`) — a path condition on
+  any other type matches nothing and silently returns zero results.
 - **Phase 3 — tags (and optionally category).** Add a `tags: list[str]` payload field in
   `processor.py`, propagate Nextcloud system tags during scanning, trigger a re-index, then wire
   `NcSelectTags` (`MatchAny` over tags). Re-index cost lives here, isolated from the cheap wins.
@@ -124,17 +198,32 @@ request model, not the algorithm.
 ### 3. Frontend: filter chips, structured payload
 
 The Astrolabe app adds filter controls to the existing collapsible advanced panel and renders each
-**active** filter as a closable `NcChip` (the same component Nextcloud Unified Search uses):
+**active** filter as a closable chip:
 
-- Modified-date range → `NcDateTimePicker type="datetime-range"` (model is `[Date, Date]`).
+- Modified-date range → two native `<input type="datetime-local">` fields (a local wall-clock
+  picker the browser validates), serialized to RFC 3339 UTC via `Date.toISOString()`. Phase 1
+  deliberately uses native inputs + lightweight CSS chips rather than taking a hard dependency on a
+  specific `@nextcloud/vue` component version (`NcDateTimePicker` / `NcChip` remain a later option);
+  this matches the component's existing native `<input type="range">` controls.
 - Doc types → existing checkbox grid, now also echoed as chips.
 - (Phase 2/3) path → `NcFilePicker`; tags → `NcSelectTags :fetch-tags`.
 
-The `/apps/astrolabe/api/search` endpoint moves from `GET` with query params to **`POST` with a JSON
-body**, because the filter set is structured and multi-valued and will keep growing. Dates are sent
-as **Unix seconds (UTC)** to match the `modified_at` payload representation exactly — no timezone or
-string-parsing ambiguity crosses the wire. Empty or partially-filled filters are omitted from the
-body rather than sent as nulls.
+Dates cross the wire as **RFC 3339 / ISO 8601 strings** (e.g. `"2026-01-01T00:00:00Z"`) — the format
+Nextcloud's date pickers and Unified Search use. The MCP/HTTP boundary parses RFC 3339 (and bare
+Unix seconds, for resilience) to int Unix seconds via `parse_modified_timestamp` before filtering,
+so the friendly wire format never forces a re-index of the integer `modified_at` payload.
+
+**Transport.** Phase 1 keeps the existing `GET /apps/astrolabe/api/search`: the two scalar date
+strings ride as query params alongside the already comma-joined `doc_types`, so no transport change
+is needed yet. The move to **`POST` with a JSON body** is deferred to Phase 2, when the filter set
+becomes genuinely structured/multi-valued (path + tags) and outgrows query params.
+
+The **Astrolabe UI validates the date range client-side** — the native picker constrains each field
+to a real datetime, and `performSearch` rejects an *after > before* range before issuing the request
+(`McpServerClient` forwards the RFC 3339 strings; `ApiController` re-validates and returns a 400 on a
+bad/inverted range). The server-side `parse_modified_timestamp` + cross-field guard is the
+authoritative backstop. (Astrolabe-side work is tracked on Deck card **#177**, label
+`repo:astrolabe`.)
 
 ## Consequences
 
@@ -150,8 +239,9 @@ body rather than sent as nulls.
 
 - Path and tag filters require index work (a payload index; a new payload field + full re-index)
   that this ADR explicitly defers — the readiness table makes that cost visible rather than implicit.
-- Moving `/api/search` to POST is a breaking change to that endpoint's contract; the PHP app and the
-  MCP backend must ship together.
+- The eventual `/api/search` GET→POST migration (Phase 2) will be a breaking change to that
+  endpoint's contract; the PHP app and the MCP backend must ship together when it lands. Phase 1
+  avoids it by keeping GET.
 - Pre-fusion filtering on a very selective `Range` can still under-fill `limit` if the candidate
   pool (`limit * 2`) is exhausted; if this proves a problem we revisit the prefetch multiplier
   rather than filtering post-fusion.
diff --git a/nextcloud_mcp_server/api/visualization.py b/nextcloud_mcp_server/api/visualization.py
index 736d9858..17a37e5b 100644
--- a/nextcloud_mcp_server/api/visualization.py
+++ b/nextcloud_mcp_server/api/visualization.py
@@ -37,7 +37,10 @@ from nextcloud_mcp_server.search.context import (
     get_chunk_with_context,
 )
 from nextcloud_mcp_server.search.verification import verify_search_results
-from nextcloud_mcp_server.utils.validation import is_valid_nextcloud_doc_id
+from nextcloud_mcp_server.utils.validation import (
+    is_valid_nextcloud_doc_id,
+    parse_modified_timestamp,
+)
 from nextcloud_mcp_server.vector.oauth_sync import (
     NotProvisionedError,
     get_user_client_basic_auth,
@@ -198,6 +201,22 @@ async def unified_search(request: Request) -> JSONResponse:
                 1.0,
                 "score_threshold",
             )
+
+            # ADR-027 modified-date range filter. Accepts RFC 3339 / ISO 8601
+            # datetimes or Unix seconds; normalized to int Unix seconds for the
+            # numeric Range filter. Absent bound ⇒ open-ended.
+            modified_after = parse_modified_timestamp(
+                body.get("modified_after"), param_name="modified_after"
+            )
+            modified_before = parse_modified_timestamp(
+                body.get("modified_before"), param_name="modified_before"
+            )
+            if (
+                modified_after is not None
+                and modified_before is not None
+                and modified_after > modified_before
+            ):
+                raise ValueError("modified_after must be <= modified_before")
         except ValueError as e:
             return JSONResponse({"error": str(e)}, status_code=400)
 
@@ -245,6 +264,8 @@ async def unified_search(request: Request) -> JSONResponse:
                                 limit=search_limit,
                                 doc_type=doc_type,
                                 accessible_owners=owners,
+                                modified_after=modified_after,
+                                modified_before=modified_before,
                             )
                         )
                 # Sort, then cap to a fixed over-fetch budget before the result
@@ -262,6 +283,8 @@ async def unified_search(request: Request) -> JSONResponse:
                     user_id=user_id,
                     limit=search_limit,
                     accessible_owners=owners,
+                    modified_after=modified_after,
+                    modified_before=modified_before,
                 )
             return results
 
@@ -413,6 +436,17 @@ async def vector_search(request: Request) -> JSONResponse:
         limit = min(body.get("limit", 10), 50)  # Enforce max limit
         include_pca = body.get("include_pca", True)
         doc_types = body.get("doc_types")  # Optional list of document types
+        # ADR-027 modified-date range filter. Accepts RFC 3339 / ISO 8601
+        # datetimes or Unix seconds; normalized to int Unix seconds. None ⇒ open.
+        try:
+            modified_after = parse_modified_timestamp(
+                body.get("modified_after"), param_name="modified_after"
+            )
+            modified_before = parse_modified_timestamp(
+                body.get("modified_before"), param_name="modified_before"
+            )
+        except ValueError as e:
+            return JSONResponse({"error": str(e)}, status_code=400)
 
         if not query:
             return JSONResponse(
@@ -420,6 +454,16 @@ async def vector_search(request: Request) -> JSONResponse:
                 status_code=400,
             )
 
+        if (
+            modified_after is not None
+            and modified_before is not None
+            and modified_after > modified_before
+        ):
+            return JSONResponse(
+                {"error": "modified_after must be <= modified_before"},
+                status_code=400,
+            )
+
         # Validate algorithm
         valid_algorithms = {"semantic", "bm25", "hybrid"}
         if algorithm not in valid_algorithms:
@@ -455,6 +499,8 @@ async def vector_search(request: Request) -> JSONResponse:
                                 limit=limit,
                                 doc_type=doc_type,
                                 accessible_owners=owners,
+                                modified_after=modified_after,
+                                modified_before=modified_before,
                             )
                         )
                 # Sort merged results by score and limit
@@ -467,6 +513,8 @@ async def vector_search(request: Request) -> JSONResponse:
                     user_id=user_id,
                     limit=limit,
                     accessible_owners=owners,
+                    modified_after=modified_after,
+                    modified_before=modified_before,
                 )
             return results
 
diff --git a/nextcloud_mcp_server/auth/viz_routes.py b/nextcloud_mcp_server/auth/viz_routes.py
index 849a348b..57e9b6bb 100644
--- a/nextcloud_mcp_server/auth/viz_routes.py
+++ b/nextcloud_mcp_server/auth/viz_routes.py
@@ -39,7 +39,10 @@ from nextcloud_mcp_server.search.context import (
     get_chunk_with_context,
 )
 from nextcloud_mcp_server.search.verification import verify_search_results
-from nextcloud_mcp_server.utils.validation import is_valid_nextcloud_doc_id
+from nextcloud_mcp_server.utils.validation import (
+    is_valid_nextcloud_doc_id,
+    parse_modified_timestamp,
+)
 from nextcloud_mcp_server.vector.oauth_sync import (
     NotProvisionedError,
     get_user_client_basic_auth,
@@ -141,6 +144,31 @@ async def vector_visualization_search(request: Request) -> JSONResponse:
     doc_types_param = request.query_params.get("doc_types", "")
     doc_types = doc_types_param.split(",") if doc_types_param else None
 
+    # Parse ADR-027 modified-date range filter. Accepts RFC 3339 / ISO 8601
+    # datetimes or Unix seconds; normalized to int Unix seconds. Absent ⇒
+    # open-ended. Unparseable input or an inverted range returns 400.
+    try:
+        modified_after = parse_modified_timestamp(
+            request.query_params.get("modified_after"), param_name="modified_after"
+        )
+        modified_before = parse_modified_timestamp(
+            request.query_params.get("modified_before"), param_name="modified_before"
+        )
+    except ValueError as exc:
+        return JSONResponse(
+            {"success": False, "error": str(exc)},
+            status_code=400,
+        )
+    if (
+        modified_after is not None
+        and modified_before is not None
+        and modified_after > modified_before
+    ):
+        return JSONResponse(
+            {"success": False, "error": "modified_after must be <= modified_before"},
+            status_code=400,
+        )
+
     logger.info(
         "Viz search: user=%s, query='%s', algorithm=%s, fusion=%s, limit=%s, doc_types=%s",
         username,
@@ -202,6 +230,8 @@ async def vector_visualization_search(request: Request) -> JSONResponse:
                         doc_type=None,  # Search all types
                         score_threshold=score_threshold,
                         accessible_owners=accessible_owners,
+                        modified_after=modified_after,
+                        modified_before=modified_before,
                     )
                 all_results.extend(unverified_results)
             else:
@@ -222,6 +252,8 @@ async def vector_visualization_search(request: Request) -> JSONResponse:
                             doc_type=doc_type,
                             score_threshold=score_threshold,
                             accessible_owners=accessible_owners,
+                            modified_after=modified_after,
+                            modified_before=modified_before,
                         )
                     all_results.extend(unverified_results)
                 # Sort by score, then cap to the same limit*2 over-fetch budget
diff --git a/nextcloud_mcp_server/search/access_filter.py b/nextcloud_mcp_server/search/access_filter.py
index 70607afc..35d2866b 100644
--- a/nextcloud_mcp_server/search/access_filter.py
+++ b/nextcloud_mcp_server/search/access_filter.py
@@ -29,7 +29,16 @@ import time
 from collections import OrderedDict
 from typing import Any, Protocol
 
-from qdrant_client.models import Condition, FieldCondition, Filter, MatchAny, MatchValue
+from qdrant_client.models import (
+    Condition,
+    FieldCondition,
+    Filter,
+    MatchAny,
+    MatchValue,
+    Range,
+)
+
+from nextcloud_mcp_server.vector.placeholder import get_placeholder_filter
 
 logger = logging.getLogger(__name__)
 
@@ -177,3 +186,60 @@ def build_ownership_filter(
             0, FieldCondition(key="owner_id", match=MatchAny(any=other_owners))
         )
     return Filter(should=conditions)
+
+
+def build_base_filter_conditions(
+    user_id: str,
+    accessible_owners: list[str] | None = None,
+    doc_type: str | None = None,
+    modified_after: int | None = None,
+    modified_before: int | None = None,
+) -> list[Condition]:
+    """Build the common ``must`` conditions shared by every search algorithm.
+
+    This is the single place the structured-filter contract (ADR-027) lives, so
+    both the BM25-hybrid (MCP tool) and dense-only (visualization/API) algorithms
+    apply identical placeholder/ACL/doc_type/date filtering. Each algorithm wraps
+    the returned list in ``Filter(must=...)`` and may append its own additive
+    conditions afterward (e.g. the dense algorithm's opt-in ACL pre-filter).
+
+    The conditions, in order:
+
+    1. ``get_placeholder_filter()`` — exclude in-flight placeholder points.
+    2. ``build_ownership_filter(...)`` — ACL-aware ``owner_id``/``user_id`` scope.
+    3. ``doc_type`` exact match — only when ``doc_type`` is truthy.
+    4. ``modified_at`` range — only when at least one bound is given.
+
+    Args:
+        user_id: Querying user.
+        accessible_owners: Owner UIDs the user can read (see
+            ``build_ownership_filter``). ``None`` ⇒ self-only.
+        doc_type: Optional single document-type filter.
+        modified_after: Inclusive lower bound on ``modified_at`` (Unix seconds).
+        modified_before: Inclusive upper bound on ``modified_at`` (Unix seconds).
+
+    Returns:
+        A list of Qdrant ``Condition`` objects for a parent ``must`` clause.
+    """
+    conditions: list[Condition] = [
+        get_placeholder_filter(),
+        build_ownership_filter(user_id, accessible_owners),
+    ]
+
+    if doc_type:
+        conditions.append(
+            FieldCondition(key="doc_type", match=MatchValue(value=doc_type))
+        )
+
+    # ``Range`` treats ``None`` bounds as open-ended, so the same condition serves
+    # after-only, before-only, and both-bounds queries. Appended only when at
+    # least one bound is set so unfiltered searches add no condition.
+    if modified_after is not None or modified_before is not None:
+        conditions.append(
+            FieldCondition(
+                key="modified_at",
+                range=Range(gte=modified_after, lte=modified_before),
+            )
+        )
+
+    return conditions
diff --git a/nextcloud_mcp_server/search/algorithms.py b/nextcloud_mcp_server/search/algorithms.py
index bbd22341..ab168fe7 100644
--- a/nextcloud_mcp_server/search/algorithms.py
+++ b/nextcloud_mcp_server/search/algorithms.py
@@ -289,6 +289,8 @@ class SearchAlgorithm(ABC):
         doc_type: str | None = None,
         *,
         accessible_owners: list[str] | None = None,
+        modified_after: int | None = None,
+        modified_before: int | None = None,
         **kwargs: Any,
     ) -> list[SearchResult]:
         """Execute search with the given parameters.
@@ -304,6 +306,12 @@ class SearchAlgorithm(ABC):
                 buried in ``**kwargs`` — so a misspelled keyword is a type error
                 instead of a silent fall back to self-only scope. ``None`` means
                 self-only (``[user_id]``).
+            modified_after: Optional inclusive lower bound on the document's
+                ``modified_at`` payload field (Unix seconds, UTC). Declared
+                explicitly for the same discoverability/type-safety reason as
+                ``accessible_owners`` (ADR-027). ``None`` ⇒ open-ended.
+            modified_before: Optional inclusive upper bound on ``modified_at``
+                (Unix seconds, UTC). ``None`` ⇒ open-ended.
             **kwargs: Algorithm-specific parameters
 
         Returns:
diff --git a/nextcloud_mcp_server/search/bm25_hybrid.py b/nextcloud_mcp_server/search/bm25_hybrid.py
index ba83d1ac..e313aa2d 100644
--- a/nextcloud_mcp_server/search/bm25_hybrid.py
+++ b/nextcloud_mcp_server/search/bm25_hybrid.py
@@ -4,19 +4,18 @@ import logging
 from typing import Any
 
 from qdrant_client import models
-from qdrant_client.models import FieldCondition, Filter, MatchValue
+from qdrant_client.models import Filter
 
 from nextcloud_mcp_server.config import get_settings
 from nextcloud_mcp_server.embedding import get_bm25_service, get_embedding_service
 from nextcloud_mcp_server.observability.metrics import record_qdrant_operation
 from nextcloud_mcp_server.observability.tracing import trace_operation
-from nextcloud_mcp_server.search.access_filter import build_ownership_filter
+from nextcloud_mcp_server.search.access_filter import build_base_filter_conditions
 from nextcloud_mcp_server.search.algorithms import (
     SearchAlgorithm,
     SearchResult,
     build_search_result_from_point,
 )
-from nextcloud_mcp_server.vector.placeholder import get_placeholder_filter
 from nextcloud_mcp_server.vector.qdrant_client import get_qdrant_client
 
 logger = logging.getLogger(__name__)
@@ -73,6 +72,8 @@ class BM25HybridSearchAlgorithm(SearchAlgorithm):
         doc_type: str | None = None,
         *,
         accessible_owners: list[str] | None = None,
+        modified_after: int | None = None,
+        modified_before: int | None = None,
         **kwargs: Any,
     ) -> list[SearchResult]:
         """
@@ -94,6 +95,10 @@ class BM25HybridSearchAlgorithm(SearchAlgorithm):
             accessible_owners: Owner UIDs the user can read (self + share
                 senders), pre-computed by the caller from the OCS Sharing API.
                 Defaults to ``[user_id]`` (self-only) when ``None``.
+            modified_after: Inclusive lower bound on ``modified_at`` (Unix
+                seconds, UTC); ``None`` ⇒ open-ended (ADR-027).
+            modified_before: Inclusive upper bound on ``modified_at`` (Unix
+                seconds, UTC); ``None`` ⇒ open-ended (ADR-027).
             **kwargs: Additional parameters (score_threshold override)
 
         Returns:
@@ -134,20 +139,16 @@ class BM25HybridSearchAlgorithm(SearchAlgorithm):
             len(sparse_embedding["indices"]),
         )
 
-        # Build Qdrant filter
-        filter_conditions = [
-            get_placeholder_filter(),  # Always exclude placeholders from user-facing queries
-            build_ownership_filter(user_id, accessible_owners),
-        ]
-
-        # Add doc_type filter if specified
-        if doc_type:
-            filter_conditions.append(
-                FieldCondition(
-                    key="doc_type",
-                    match=MatchValue(value=doc_type),
-                )
-            )
+        # Build Qdrant filter (placeholder + ACL + doc_type + modified_at range).
+        # Shared with the dense-only SemanticSearchAlgorithm via the common
+        # ADR-027 helper so every search surface applies one filter contract.
+        filter_conditions = build_base_filter_conditions(
+            user_id=user_id,
+            accessible_owners=accessible_owners,
+            doc_type=doc_type,
+            modified_after=modified_after,
+            modified_before=modified_before,
+        )
 
         query_filter = Filter(must=filter_conditions)
 
diff --git a/nextcloud_mcp_server/search/semantic.py b/nextcloud_mcp_server/search/semantic.py
index 51fdf55c..46fdea49 100644
--- a/nextcloud_mcp_server/search/semantic.py
+++ b/nextcloud_mcp_server/search/semantic.py
@@ -3,20 +3,19 @@
 import logging
 from typing import Any
 
-from qdrant_client.models import FieldCondition, Filter, MatchAny, MatchValue
+from qdrant_client.models import FieldCondition, Filter, MatchAny
 
 from nextcloud_mcp_server.acl_hash import accessible_hash_set
 from nextcloud_mcp_server.config import get_settings
 from nextcloud_mcp_server.embedding import get_embedding_service
 from nextcloud_mcp_server.observability.metrics import record_qdrant_operation
-from nextcloud_mcp_server.search.access_filter import build_ownership_filter
+from nextcloud_mcp_server.search.access_filter import build_base_filter_conditions
 from nextcloud_mcp_server.search.algorithms import (
     SearchAlgorithm,
     SearchResult,
     build_search_result_from_point,
 )
 from nextcloud_mcp_server.vector.payload_keys import ACL_HASH
-from nextcloud_mcp_server.vector.placeholder import get_placeholder_filter
 from nextcloud_mcp_server.vector.qdrant_client import get_qdrant_client
 
 logger = logging.getLogger(__name__)
@@ -53,6 +52,8 @@ class SemanticSearchAlgorithm(SearchAlgorithm):
         doc_type: str | None = None,
         *,
         accessible_owners: list[str] | None = None,
+        modified_after: int | None = None,
+        modified_before: int | None = None,
         **kwargs: Any,
     ) -> list[SearchResult]:
         """Execute semantic search using vector similarity.
@@ -73,6 +74,10 @@ class SemanticSearchAlgorithm(SearchAlgorithm):
             accessible_owners: Owner UIDs the user can read (self + share
                 senders), pre-computed by the caller from the OCS Sharing API.
                 Defaults to ``[user_id]`` (self-only) when ``None``.
+            modified_after: Inclusive lower bound on ``modified_at`` (Unix
+                seconds, UTC); ``None`` ⇒ open-ended (ADR-027).
+            modified_before: Inclusive upper bound on ``modified_at`` (Unix
+                seconds, UTC); ``None`` ⇒ open-ended (ADR-027).
             **kwargs:
                 - score_threshold (float): override the instance default
 
@@ -103,20 +108,17 @@ class SemanticSearchAlgorithm(SearchAlgorithm):
             "Generated embedding for query (dimension=%s)", len(query_embedding)
         )
 
-        # Build Qdrant filter
-        filter_conditions = [
-            get_placeholder_filter(),  # Always exclude placeholders from user-facing queries
-            build_ownership_filter(user_id, accessible_owners),
-        ]
-
-        # Add doc_type filter if specified
-        if doc_type:
-            filter_conditions.append(
-                FieldCondition(
-                    key="doc_type",
-                    match=MatchValue(value=doc_type),
-                )
-            )
+        # Build Qdrant filter (placeholder + ACL + doc_type + modified_at range).
+        # Shared with BM25HybridSearchAlgorithm via the common ADR-027 helper so
+        # the dense-only (API/visualization) and hybrid (MCP tool) paths apply
+        # one filter contract.
+        filter_conditions = build_base_filter_conditions(
+            user_id=user_id,
+            accessible_owners=accessible_owners,
+            doc_type=doc_type,
+            modified_after=modified_after,
+            modified_before=modified_before,
+        )
 
         # ACL pre-filter (design §11), opt-in via ACL_PREFILTER_ENABLED and OFF
         # by default. Additive `must` condition — it can only narrow results,
diff --git a/nextcloud_mcp_server/server/semantic.py b/nextcloud_mcp_server/server/semantic.py
index ed651a29..abf37fe8 100644
--- a/nextcloud_mcp_server/server/semantic.py
+++ b/nextcloud_mcp_server/server/semantic.py
@@ -1,6 +1,7 @@
 """Semantic search MCP tools using vector database."""
 
 import logging
+from typing import Annotated
 
 import anyio
 from httpx import RequestError
@@ -16,6 +17,7 @@ from mcp.types import (
     TextContent,
     ToolAnnotations,
 )
+from pydantic import Field
 from qdrant_client.models import Filter
 
 from nextcloud_mcp_server.auth import require_scopes
@@ -34,6 +36,7 @@ from nextcloud_mcp_server.search.access_filter import list_accessible_owners
 from nextcloud_mcp_server.search.bm25_hybrid import BM25HybridSearchAlgorithm
 from nextcloud_mcp_server.search.context import get_chunk_with_context
 from nextcloud_mcp_server.search.verification import verify_search_results
+from nextcloud_mcp_server.utils.validation import parse_modified_timestamp
 from nextcloud_mcp_server.vector.placeholder import get_placeholder_filter
 from nextcloud_mcp_server.vector.qdrant_client import get_qdrant_client
 
@@ -55,12 +58,32 @@ def configure_semantic_tools(mcp: FastMCP):
     async def nc_semantic_search(
         query: str,
         ctx: Context,
-        limit: int = 10,
+        limit: Annotated[int, Field(ge=1, le=100)] = 10,
         doc_types: list[str] | None = None,
-        score_threshold: float = 0.0,
+        score_threshold: Annotated[float, Field(ge=0.0, le=1.0)] = 0.0,
         fusion: str = "rrf",
         include_context: bool = False,
-        context_chars: int = 300,
+        context_chars: Annotated[int, Field(ge=0)] = 300,
+        modified_after: Annotated[
+            str | int | None,
+            Field(
+                description=(
+                    "Only return documents modified at or after this time. "
+                    "RFC 3339 / ISO 8601 datetime (e.g. '2026-01-01T00:00:00Z') "
+                    "or Unix seconds. None = no lower bound."
+                ),
+            ),
+        ] = None,
+        modified_before: Annotated[
+            str | int | None,
+            Field(
+                description=(
+                    "Only return documents modified at or before this time. "
+                    "RFC 3339 / ISO 8601 datetime or Unix seconds. "
+                    "None = no upper bound."
+                ),
+            ),
+        ] = None,
     ) -> SemanticSearchResponse:
         """
         Search Nextcloud content using BM25 hybrid search with cross-app support.
@@ -86,6 +109,13 @@ def configure_semantic_tools(mcp: FastMCP):
                    DBSF: Uses distribution-based normalization, may better balance different score ranges
             include_context: Whether to expand results with surrounding context (default: False)
             context_chars: Number of characters to include before/after matched chunk (default: 300)
+            modified_after: Only return documents whose last-modified time is at or after this
+                instant. Accepts an RFC 3339 / ISO 8601 datetime (e.g. "2026-01-01T00:00:00Z";
+                a naive datetime is treated as UTC) or Unix seconds. None = no lower bound
+                (default).
+            modified_before: Only return documents whose last-modified time is at or before this
+                instant. Same formats as modified_after. None = no upper bound (default). Must be
+                >= modified_after when both are supplied.
 
         Returns:
             SemanticSearchResponse with matching documents ranked by fusion scores.
@@ -122,6 +152,40 @@ def configure_semantic_tools(mcp: FastMCP):
                 )
             )
 
+        # Normalize the RFC 3339 / Unix-seconds date bounds to int Unix seconds
+        # for the numeric ``modified_at`` Range filter (ADR-027). A bad format
+        # surfaces as a clean McpError rather than a 500.
+        try:
+            modified_after_ts = parse_modified_timestamp(
+                modified_after, param_name="modified_after"
+            )
+            modified_before_ts = parse_modified_timestamp(
+                modified_before, param_name="modified_before"
+            )
+        except ValueError as exc:
+            raise McpError(ErrorData(code=-1, message=str(exc))) from exc
+
+        # Cross-field invariant: a per-parameter pydantic ``Field`` constraint
+        # (validated by FastMCP from the signature) bounds each date on its own
+        # but cannot express the relationship between them. Guard it here so an
+        # inverted range surfaces a clean McpError rather than silently
+        # returning zero results (ADR-027).
+        if (
+            modified_after_ts is not None
+            and modified_before_ts is not None
+            and modified_after_ts > modified_before_ts
+        ):
+            raise McpError(
+                ErrorData(
+                    code=-1,
+                    message=(
+                        "modified_after must be <= modified_before "
+                        f"(got modified_after={modified_after!r}, "
+                        f"modified_before={modified_before!r})"
+                    ),
+                )
+            )
+
         # Expand the caller's identity to every owner whose content they
         # have read access to via Nextcloud shares. Lets a user find files
         # owners have shared with them without having to re-index those
@@ -166,6 +230,8 @@ def configure_semantic_tools(mcp: FastMCP):
                     doc_type=None,  # Signal to search all types
                     score_threshold=score_threshold,
                     accessible_owners=accessible_owners,
+                    modified_after=modified_after_ts,
+                    modified_before=modified_before_ts,
                 )
                 all_results.extend(unverified_results)
             else:
@@ -191,6 +257,8 @@ def configure_semantic_tools(mcp: FastMCP):
                         doc_type=dtype,
                         score_threshold=score_threshold,
                         accessible_owners=accessible_owners,
+                        modified_after=modified_after_ts,
+                        modified_before=modified_before_ts,
                     )
                     all_results.extend(unverified_results)
 
diff --git a/nextcloud_mcp_server/utils/validation.py b/nextcloud_mcp_server/utils/validation.py
index 2f9d0287..27f2927d 100644
--- a/nextcloud_mcp_server/utils/validation.py
+++ b/nextcloud_mcp_server/utils/validation.py
@@ -1,6 +1,7 @@
 """Shared validators for primitive types crossing system boundaries."""
 
 import re
+from datetime import datetime, timezone
 
 # Nextcloud object IDs are unsigned ints from MySQL AUTO_INCREMENT, which
 # starts at 1. Restrict to ASCII positive integers to exclude Unicode digit
@@ -9,7 +10,77 @@ import re
 # and leading zeros.
 _NEXTCLOUD_DOC_ID_RE = re.compile(r"^[1-9][0-9]*$")
 
+# A bare non-negative integer string is accepted as Unix seconds (the
+# pre-RFC-3339 wire format) so older callers keep working.
+_UNIX_SECONDS_RE = re.compile(r"^[0-9]+$")
+
 
 def is_valid_nextcloud_doc_id(value: str) -> bool:
     """True iff `value` is the str form of a positive ASCII integer (>= 1)."""
     return bool(_NEXTCLOUD_DOC_ID_RE.fullmatch(value))
+
+
+def parse_modified_timestamp(
+    value: str | int | float | None,
+    *,
+    param_name: str = "modified_at",
+) -> int | None:
+    """Normalize a search date-filter bound to an int Unix-second timestamp.
+
+    ADR-027: callers (the MCP tool, the ``/api/v1`` search endpoints, and the
+    visualization route) accept **RFC 3339 / ISO 8601** datetimes at the
+    boundary — the ergonomic, Nextcloud-Unified-Search-style format — while the
+    ``modified_at`` Qdrant payload stays an int Unix-second timestamp (a
+    cross-app normalization done by the scanner). This converts the former to
+    the latter so the numeric ``Range`` filter and INTEGER payload index work
+    without re-indexing.
+
+    Accepts:
+
+    - ``None`` / empty string ⇒ ``None`` (open-ended bound).
+    - ``int`` / ``float`` ⇒ truncated to int seconds.
+    - A bare non-negative integer string ⇒ Unix seconds (legacy wire format).
+    - An RFC 3339 / ISO 8601 string, e.g. ``"2026-01-01T00:00:00Z"`` or
+      ``"2026-01-01T00:00:00+02:00"``. A naive datetime (no offset) is assumed
+      to be UTC, matching the payload representation.
+
+    Args:
+        value: The raw bound from the request.
+        param_name: Field name used in error messages.
+
+    Returns:
+        Int Unix-second timestamp (UTC), or ``None`` for an open bound.
+
+    Raises:
+        ValueError: If the value is negative or cannot be parsed.
+    """
+    if value is None:
+        return None
+    # bool is an int subclass — reject it explicitly so True/False aren't
+    # silently read as 1/0 seconds.
+    if isinstance(value, bool):
+        raise ValueError(f"{param_name} must be a datetime string or Unix seconds")
+    if isinstance(value, (int, float)):
+        seconds = int(value)
+        if seconds < 0:
+            raise ValueError(f"{param_name} must be >= 0, got {seconds}")
+        return seconds
+    if isinstance(value, str):
+        text = value.strip()
+        if not text:
+            return None
+        if _UNIX_SECONDS_RE.fullmatch(text):
+            return int(text)
+        # RFC 3339 / ISO 8601. ``fromisoformat`` accepts a trailing "Z" only on
+        # Python 3.11+; normalize it for safety across versions.
+        try:
+            parsed = datetime.fromisoformat(text.replace("Z", "+00:00"))
+        except ValueError as exc:
+            raise ValueError(
+                f"{param_name} must be an RFC 3339 datetime or Unix seconds, "
+                f"got {value!r}"
+            ) from exc
+        if parsed.tzinfo is None:
+            parsed = parsed.replace(tzinfo=timezone.utc)
+        return int(parsed.timestamp())
+    raise ValueError(f"{param_name} must be a datetime string or Unix seconds")
diff --git a/nextcloud_mcp_server/vector/qdrant_client.py b/nextcloud_mcp_server/vector/qdrant_client.py
index 9f110e43..b2df4873 100644
--- a/nextcloud_mcp_server/vector/qdrant_client.py
+++ b/nextcloud_mcp_server/vector/qdrant_client.py
@@ -52,6 +52,16 @@ _PAYLOAD_INDEX_FIELDS: dict[str, PayloadSchemaType] = {
     "chunk_index": PayloadSchemaType.INTEGER,
     "chunk_start_offset": PayloadSchemaType.INTEGER,
     "chunk_end_offset": PayloadSchemaType.INTEGER,
+    # modified_at is the ADR-027 date-range filter field: searches apply
+    # Range(key="modified_at", gte=..., lte=...) (see
+    # search/access_filter.build_base_filter_conditions). Qdrant requires a
+    # payload index to evaluate a Range efficiently — without one every dated
+    # query full-scans the collection (and 400s on Qdrant Cloud strict mode).
+    # INTEGER (not FLOAT): modified_at is an int Unix-second timestamp on every
+    # point (vector/processor.py, vector/placeholder.py). _ensure_payload_indexes
+    # is idempotent, so existing collections gain this index at startup with no
+    # content re-index and no operator action.
+    "modified_at": PayloadSchemaType.INTEGER,
 }
 
 # Sentinel point that records "this collection has been backfilled to str
diff --git a/tests/unit/search/test_access_filter.py b/tests/unit/search/test_access_filter.py
index bf52b888..b708aec3 100644
--- a/tests/unit/search/test_access_filter.py
+++ b/tests/unit/search/test_access_filter.py
@@ -5,9 +5,11 @@ from __future__ import annotations
 from unittest.mock import AsyncMock
 
 import pytest
+from qdrant_client.models import FieldCondition, Range
 
 from nextcloud_mcp_server.search import access_filter
 from nextcloud_mcp_server.search.access_filter import (
+    build_base_filter_conditions,
     build_ownership_filter,
     clear_accessible_owners_cache,
     list_accessible_owners,
@@ -186,3 +188,75 @@ class TestBuildOwnershipFilter:
         (user_branch,) = flt.should
         assert user_branch.key == "user_id"
         assert user_branch.match.value == "alice"
+
+
+class TestBuildBaseFilterConditions:
+    """The shared ADR-027 filter contract used by both search algorithms."""
+
+    @pytest.mark.unit
+    def test_minimal_is_placeholder_plus_ownership(self) -> None:
+        # No doc_type, no date bounds -> exactly placeholder + ownership.
+        conditions = build_base_filter_conditions("alice", None)
+        assert len(conditions) == 2
+        # No modified_at Range condition present.
+        assert not any(
+            isinstance(c, FieldCondition) and c.key == "modified_at" for c in conditions
+        )
+
+    @pytest.mark.unit
+    def test_doc_type_appends_match_condition(self) -> None:
+        conditions = build_base_filter_conditions("alice", None, doc_type="note")
+        doc_type_conds = [
+            c
+            for c in conditions
+            if isinstance(c, FieldCondition) and c.key == "doc_type"
+        ]
+        assert len(doc_type_conds) == 1
+        assert doc_type_conds[0].match.value == "note"
+
+    @pytest.mark.unit
+    @pytest.mark.parametrize(
+        "after,before,expected_gte,expected_lte",
+        [
+            (100, 200, 100, 200),
+            (100, None, 100, None),  # after-only
+            (None, 200, None, 200),  # before-only
+        ],
+    )
+    def test_modified_at_range_appended(
+        self, after, before, expected_gte, expected_lte
+    ) -> None:
+        conditions = build_base_filter_conditions(
+            "alice", None, modified_after=after, modified_before=before
+        )
+        range_conds = [
+            c
+            for c in conditions
+            if isinstance(c, FieldCondition) and c.key == "modified_at"
+        ]
+        assert len(range_conds) == 1
+        rng = range_conds[0].range
+        assert isinstance(rng, Range)
+        assert rng.gte == expected_gte
+        assert rng.lte == expected_lte
+
+    @pytest.mark.unit
+    def test_no_range_when_both_bounds_none(self) -> None:
+        conditions = build_base_filter_conditions(
+            "alice", None, modified_after=None, modified_before=None
+        )
+        assert not any(
+            isinstance(c, FieldCondition) and c.key == "modified_at" for c in conditions
+        )
+
+    @pytest.mark.unit
+    def test_all_filters_compose(self) -> None:
+        # placeholder + ownership + doc_type + modified_at range = 4 conditions.
+        conditions = build_base_filter_conditions(
+            "alice",
+            ["alice", "bob"],
+            doc_type="file",
+            modified_after=100,
+            modified_before=200,
+        )
+        assert len(conditions) == 4
diff --git a/tests/unit/utils/test_validation.py b/tests/unit/utils/test_validation.py
index 8e1f38ed..48eb55ac 100644
--- a/tests/unit/utils/test_validation.py
+++ b/tests/unit/utils/test_validation.py
@@ -1,8 +1,13 @@
 """Unit tests for shared boundary validators."""
 
+from datetime import datetime, timezone
+
 import pytest
 
-from nextcloud_mcp_server.utils.validation import is_valid_nextcloud_doc_id
+from nextcloud_mcp_server.utils.validation import (
+    is_valid_nextcloud_doc_id,
+    parse_modified_timestamp,
+)
 
 
 @pytest.mark.unit
@@ -52,3 +57,62 @@ def test_accepts_positive_ascii_integers(value):
 def test_rejects_invalid_doc_ids(value, reason):
     """Reject empty/zero/leading-zero/non-ASCII/non-digit inputs."""
     assert is_valid_nextcloud_doc_id(value) is False, f"should reject: {reason}"
+
+
+# parse_modified_timestamp (ADR-027) ---------------------------------------
+
+_JAN_2026_UTC = int(datetime(2026, 1, 1, tzinfo=timezone.utc).timestamp())
+
+
+@pytest.mark.unit
+@pytest.mark.parametrize(
+    "value,expected",
+    [
+        (None, None),
+        ("", None),
+        ("   ", None),
+        # RFC 3339 / ISO 8601
+        ("2026-01-01T00:00:00Z", _JAN_2026_UTC),
+        ("2026-01-01T00:00:00+00:00", _JAN_2026_UTC),
+        # +02:00 offset is two hours earlier in UTC
+        ("2026-01-01T02:00:00+02:00", _JAN_2026_UTC),
+        # Naive datetime is assumed UTC
+        ("2026-01-01T00:00:00", _JAN_2026_UTC),
+        # Date-only ISO form
+        ("2026-01-01", _JAN_2026_UTC),
+        # Bare Unix seconds (string and int) pass through
+        (str(_JAN_2026_UTC), _JAN_2026_UTC),
+        (_JAN_2026_UTC, _JAN_2026_UTC),
+        (0, 0),
+        (1767225600.9, 1767225600),  # float truncates
+    ],
+)
+def test_parse_modified_timestamp_accepts(value, expected):
+    """RFC 3339 strings, Unix seconds, and None normalize to int seconds (UTC)."""
+    assert parse_modified_timestamp(value) == expected
+
+
+@pytest.mark.unit
+@pytest.mark.parametrize(
+    "value,reason",
+    [
+        ("not-a-date", "unparseable string"),
+        ("2026-13-01T00:00:00Z", "invalid month"),
+        (-1, "negative int"),
+        (-5.0, "negative float"),
+        (True, "bool is not a timestamp"),
+        (False, "bool is not a timestamp"),
+        ([], "wrong type"),
+    ],
+)
+def test_parse_modified_timestamp_rejects(value, reason):
+    """Bad formats / negatives / bools raise ValueError (→ McpError or HTTP 400)."""
+    with pytest.raises(ValueError):
+        parse_modified_timestamp(value)
+
+
+@pytest.mark.unit
+def test_parse_modified_timestamp_error_names_param():
+    """The param_name is surfaced in the error for caller-friendly messages."""
+    with pytest.raises(ValueError, match="modified_after"):
+        parse_modified_timestamp("nope", param_name="modified_after")
diff --git a/tests/unit/vector/test_qdrant_client.py b/tests/unit/vector/test_qdrant_client.py
index cb0f5236..f5ebdcbb 100644
--- a/tests/unit/vector/test_qdrant_client.py
+++ b/tests/unit/vector/test_qdrant_client.py
@@ -73,6 +73,21 @@ def _record(point_id: int | str, doc_id: int | str | None) -> SimpleNamespace:
     return SimpleNamespace(id=point_id, payload=payload)
 
 
+# ---------------------------------------------------------------------------
+# _PAYLOAD_INDEX_FIELDS contract
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.unit
+def test_modified_at_indexed_as_integer():
+    """ADR-027: the date-range filter needs a numeric index on modified_at.
+
+    INTEGER (not FLOAT/DATETIME) because the payload stores an int Unix-second
+    timestamp; a numeric Range filters it without a content re-index.
+    """
+    assert _PAYLOAD_INDEX_FIELDS.get("modified_at") == PayloadSchemaType.INTEGER
+
+
 # ---------------------------------------------------------------------------
 # _ensure_payload_indexes
 # ---------------------------------------------------------------------------