docs: USE_DATETIME env var description

Author: Yuri Zmytrakov

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ## [Unreleased]
 
+### Added
+
+- Added `USE_DATETIME` environment variable to configure datetime search behavior in SFEOS. [#452](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/452)
+
 ### Changed
 
 - unified the type of queryables endpoint to `application/schema+json`. [#445](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/445)

README.md

@@ -247,6 +247,7 @@ You can customize additional settings in your `.env` file:
 | `STAC_ITEM_LIMIT` | Sets the environment variable for result limiting to SFEOS for the number of returned items and STAC collections. | `10` | Optional |
 | `STAC_INDEX_ASSETS` | Controls if Assets are indexed when added to Elasticsearch/Opensearch. This allows asset fields to be included in search queries. | `false` | Optional |
 | `ENV_MAX_LIMIT` | Configures the environment variable in SFEOS to override the default `MAX_LIMIT`, which controls the limit parameter for returned items and STAC collections. | `10,000` | Optional |
+| `USE_DATETIME` | Configures the datetime search behavior in SFEOS. When enabled, searches both datetime field and falls back to start_datetime/end_datetime range for items with null datetime. When disabled, searches only by start_datetime/end_datetime range. | True | Optional |
 
 > [!NOTE]
 > The variables `ES_HOST`, `ES_PORT`, `ES_USE_SSL`, `ES_VERIFY_CERTS` and `ES_TIMEOUT` apply to both Elasticsearch and OpenSearch backends, so there is no need to rename the key names to `OS_` even if you're using OpenSearch.