diff --git a/CHANGELOG.md b/CHANGELOG.md
index 21d9fd130..ae7511646 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -22,6 +22,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 - Added `id` field as secondary sort to sort config to ensure unique pagination tokens. [#421](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/421)
 - Added default environment variable `STAC_ITEM_LIMIT` to SFEOS for result limiting of returned items and STAC collections [#419](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/419)
+- Updated the `format_datetime_range` function to support milliseconds. [#423](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/423)
 
 ### Changed
 
diff --git a/stac_fastapi/core/stac_fastapi/core/datetime_utils.py b/stac_fastapi/core/stac_fastapi/core/datetime_utils.py
index 87911ac51..d5f992de8 100644
--- a/stac_fastapi/core/stac_fastapi/core/datetime_utils.py
+++ b/stac_fastapi/core/stac_fastapi/core/datetime_utils.py
@@ -17,17 +17,20 @@ def format_datetime_range(date_str: str) -> str:
     """
 
     def normalize(dt):
+        """Normalize datetime string and preserve millisecond precision."""
         dt = dt.strip()
         if not dt or dt == "..":
             return ".."
         dt_obj = rfc3339_str_to_datetime(dt)
         dt_utc = dt_obj.astimezone(timezone.utc)
-        return dt_utc.strftime("%Y-%m-%dT%H:%M:%SZ")
+        return dt_utc.isoformat(timespec="milliseconds").replace("+00:00", "Z")
 
     if not isinstance(date_str, str):
         return "../.."
+
     if "/" not in date_str:
         return f"{normalize(date_str)}/{normalize(date_str)}"
+
     try:
         start, end = date_str.split("/", 1)
     except Exception: