diff --git a/CHANGES.md b/CHANGES.md
index 07a80ad05..f4222417d 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -7,6 +7,7 @@
 - Return a `Location` header with the URI of the newly created resource for `POST /collections/{collection_id}/items` (single `Item`) and `POST /collections` in the Transaction extension, as required by the spec. `ItemCollection` payloads still return 201 without a `Location` header. The header is also added when a client returns a `Response` object directly, unless it already sets `Location` or returns an error status. ([#982](https://github.com/stac-utils/stac-fastapi/pull/982))
 - Type `PartialCollection.links` as a list so `PATCH /collections/{collection_id}` merge patches carrying a `links` array are no longer rejected with 400.
 - Declare `ItemCollection` as a valid response of `POST /collections/{collection_id}/items` (route response model and `create_item` client return type) so a successful ItemCollection create returns 201 instead of 500 when `enable_response_models` is on.
+- Return a stable `Invalid RFC3339 datetime.` error detail from `str_to_interval` for out-of-range datetime values instead of the Python-version-dependent message. ([#991](https://github.com/stac-utils/stac-fastapi/pull/991))
 
 ## [7.0.0] - 2026-09-09
 
diff --git a/stac_fastapi/types/stac_fastapi/types/rfc3339.py b/stac_fastapi/types/stac_fastapi/types/rfc3339.py
index f69ad3d87..ceba09a65 100644
--- a/stac_fastapi/types/stac_fastapi/types/rfc3339.py
+++ b/stac_fastapi/types/stac_fastapi/types/rfc3339.py
@@ -67,7 +67,12 @@ def rfc3339_str_to_datetime(s: str) -> datetime:
         raise ValueError("Invalid RFC3339 datetime.")
 
     # Parse with pyiso8601
-    return iso8601.parse_date(s)
+    try:
+        return iso8601.parse_date(s)
+    except iso8601.ParseError as e:
+        # pyiso8601 forwards the Python-version-dependent datetime error message,
+        # so the public message is fixed and the original error stays in the chain
+        raise ValueError("Invalid RFC3339 datetime.") from e
 
 
 def parse_single_date(date_str: str) -> datetime:
diff --git a/stac_fastapi/types/tests/test_rfc3339.py b/stac_fastapi/types/tests/test_rfc3339.py
index 3b1ea2c5d..396c0d6aa 100644
--- a/stac_fastapi/types/tests/test_rfc3339.py
+++ b/stac_fastapi/types/tests/test_rfc3339.py
@@ -103,6 +103,20 @@ def test_str_to_interval_with_invalid_datetime(test_input):
     )
 
 
+@pytest.mark.parametrize(
+    "test_input",
+    [
+        "2020-13-01T00:00:00Z",  # month out of range
+        "2020-01-01T25:00:00Z",  # hour out of range
+        "2020-01-01T00:00:00Z/2020-01-32T00:00:00Z",  # day out of range in interval
+    ],
+)
+def test_str_to_interval_out_of_range_detail_is_stable(test_input):
+    with pytest.raises(HTTPException) as exc_info:
+        str_to_interval(test_input)
+    assert exc_info.value.detail == "Invalid RFC3339 datetime."
+
+
 @pytest.mark.parametrize("test_input", valid_intervals)
 def test_str_to_interval_with_valid_interval(test_input):
     assert isinstance(str_to_interval(test_input), tuple), (