diff --git a/CHANGELOG.md b/CHANGELOG.md
index eb24c725..cfd88d5f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -18,6 +18,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 ### Fixed
 
 - Some mishandled cases for datetime intervals [#363](https://github.com/stac-utils/pystac-client/pull/363)
+- Parse datetimes more strictly [#364](https://github.com/stac-utils/pystac-client/pull/364)
 
 ### Removed
 
diff --git a/pystac_client/item_search.py b/pystac_client/item_search.py
index ba1295d2..a229aecc 100644
--- a/pystac_client/item_search.py
+++ b/pystac_client/item_search.py
@@ -34,9 +34,9 @@
     from pystac_client import client as _client
 
 DATETIME_REGEX = re.compile(
-    r"(?P<year>\d{4})(-(?P<month>\d{2})(-(?P<day>\d{2})"
+    r"^(?P<year>\d{4})(-(?P<month>\d{2})(-(?P<day>\d{2})"
     r"(?P<remainder>([Tt])\d{2}:\d{2}:\d{2}(\.\d+)?"
-    r"(?P<tz_info>Z|([-+])(\d{2}):(\d{2}))?)?)?)?"
+    r"(?P<tz_info>[Zz]|([-+])(\d{2}):(\d{2}))?)?)?)?$"
 )
 
 
diff --git a/tests/test_item_search.py b/tests/test_item_search.py
index 718742d0..bdfc76f4 100644
--- a/tests/test_item_search.py
+++ b/tests/test_item_search.py
@@ -276,6 +276,10 @@ def test_datetime_list_of_one_none(self) -> None:
         with pytest.raises(Exception):
             ItemSearch(url=SEARCH_URL, datetime=[None])
 
+    def test_poorly_formed_datetimes(self) -> None:
+        with pytest.raises(Exception):
+            ItemSearch(url=SEARCH_URL, datetime="2020-7/2020-8")
+
     def test_single_collection_string(self) -> None:
         # Single ID string
         search = ItemSearch(url=SEARCH_URL, collections="naip")