diff --git a/stac_fastapi/core/stac_fastapi/core/datetime_utils.py b/stac_fastapi/core/stac_fastapi/core/datetime_utils.py
index 87911ac5..d33c3027 100644
--- a/stac_fastapi/core/stac_fastapi/core/datetime_utils.py
+++ b/stac_fastapi/core/stac_fastapi/core/datetime_utils.py
@@ -17,22 +17,32 @@ 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")
+
+        if dt_obj.microsecond > 0:
+            return dt_utc.isoformat(timespec="milliseconds").replace("+00:00", "Z")
+        else:
+            return dt_utc.strftime("%Y-%m-%dT%H:%M:%SZ")
 
     if not isinstance(date_str, str):
         return "../.."
+
+    if "/" in date_str and ".." in date_str:
+        return date_str
+
     if "/" not in date_str:
-        return f"{normalize(date_str)}/{normalize(date_str)}"
+        return normalize(date_str)
+
     try:
         start, end = date_str.split("/", 1)
+        return f"{normalize(start)}/{normalize(end)}"
     except Exception:
         return "../.."
-    return f"{normalize(start)}/{normalize(end)}"
 
 
 # Borrowed from pystac - https://github.com/stac-utils/pystac/blob/f5e4cf4a29b62e9ef675d4a4dac7977b09f53c8f/pystac/utils.py#L370-L394