diff --git a/CHANGES.md b/CHANGES.md
index 6bf2c50b5..00552bf87 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+* BaseSearchGetRequest datetime validator str_to_interval not allowing GET /search requests with datetime = None ([#661](https://github.com/stac-utils/stac-fastapi/pull/661))
+
 ## [2.5.1] - 2024-04-18
 
 ### Fixed
diff --git a/stac_fastapi/types/stac_fastapi/types/rfc3339.py b/stac_fastapi/types/stac_fastapi/types/rfc3339.py
index 43baa8d53..e5a3cd7fb 100644
--- a/stac_fastapi/types/stac_fastapi/types/rfc3339.py
+++ b/stac_fastapi/types/stac_fastapi/types/rfc3339.py
@@ -46,7 +46,7 @@ def rfc3339_str_to_datetime(s: str) -> datetime:
 
 
 def str_to_interval(
-    interval: str,
+    interval: Optional[str]
 ) -> Optional[DateTimeType]:
     """Extract a tuple of datetimes from an interval string.
 
@@ -56,12 +56,18 @@ def str_to_interval(
     or end (but not both) to be open-ended with '..' or ''.
 
     Args:
-        interval (str) : The interval string to convert to a :class:`datetime.datetime`
-        tuple.
+        interval (str or None): The interval string to convert to a tuple of datetime.datetime 
+        objects, or None if no datetime is specified.
+
+    Returns:
+        Optional[DateTimeType]: A tuple of datetime.datetime objects or None if input is None.
 
     Raises:
-        ValueError: If the string is not a valid interval string.
+        ValueError: If the string is not a valid interval string and not None.
     """
+    if interval is None:
+        return None
+    
     if not interval:
         raise ValueError("Empty interval string is invalid.")
 
diff --git a/stac_fastapi/types/tests/test_rfc3339.py b/stac_fastapi/types/tests/test_rfc3339.py
index 0a402699a..9d86b6ff6 100644
--- a/stac_fastapi/types/tests/test_rfc3339.py
+++ b/stac_fastapi/types/tests/test_rfc3339.py
@@ -103,3 +103,7 @@ def test_now_functions() -> None:
     assert now1.tzinfo == timezone.utc
 
     rfc3339_str_to_datetime(now_to_rfc3339_str())
+
+def test_str_to_interval_with_none():
+    """Test that str_to_interval returns None when provided with None."""
+    assert str_to_interval(None) is None, "str_to_interval should return None when input is None"
\ No newline at end of file