fix: ensure datetime in post request is normalized

CHANGELOG.md

@@ -14,6 +14,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 ### Fixed
 
 - Fixed datetime filtering for .0Z milliseconds to preserve precision in apply_filter_datetime, ensuring only items exactly within the specified range are returned. [#535](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/535)
+- Normalize datetime in POST /search requests to match GET /search behavior. [#543](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/543)
 
 ### Removed
 

stac_fastapi/core/stac_fastapi/core/core.py

@@ -790,9 +790,10 @@ async def post_search(
                 search=search, collection_ids=search_request.collections
             )
 
+        datetime_parsed = format_datetime_range(date_str=search_request.datetime)
         try:
             search, datetime_search = self.database.apply_datetime_filter(
-                search=search, datetime=search_request.datetime
+                search=search, datetime=datetime_parsed
             )
         except (ValueError, TypeError) as e:
             # Handle invalid interval formats if return_date fails