diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0624442c..c1b4e176 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,9 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 ### Changed
 
 - Relax `requests` dependency [#87](https://github.com/stac-utils/pystac-client/pull/87)
+### Fixed
+
+- `ItemSearch` now correctly handles times without a timezone specifier [#92](https://github.com/stac-utils/pystac-client/issues/92)
 
 ## [v0.2.0] - 2021-08-04
 
diff --git a/pystac_client/item_search.py b/pystac_client/item_search.py
index 2afc8ccf..04eff243 100644
--- a/pystac_client/item_search.py
+++ b/pystac_client/item_search.py
@@ -17,9 +17,9 @@
 if TYPE_CHECKING:
     from pystac_client.client import Client
 
-DATETIME_REGEX = re.compile(
-    r"(?P<year>\d{4})(\-(?P<month>\d{2})(\-(?P<day>\d{2})"
-    r"(?P<remainder>(T|t)\d{2}:\d{2}:\d{2}(\.\d+)?(Z|([-+])(\d{2}):(\d{2})))?)?)?")
+DATETIME_REGEX = re.compile(r"(?P<year>\d{4})(\-(?P<month>\d{2})(\-(?P<day>\d{2})"
+                            r"(?P<remainder>(T|t)\d{2}:\d{2}:\d{2}(\.\d+)?"
+                            r"(?P<tz_info>Z|([-+])(\d{2}):(\d{2}))?)?)?)?")
 
 DatetimeOrTimestamp = Optional[Union[datetime_, str]]
 Datetime = Union[Tuple[str], Tuple[str, str]]
@@ -270,7 +270,10 @@ def _to_isoformat_range(component: DatetimeOrTimestamp):
                 if not match:
                     raise Exception(f"invalid datetime component: {component}")
                 elif match.group("remainder"):
-                    return component, None
+                    if match.group("tz_info"):
+                        return component, None
+                    else:
+                        return f"{component}Z", None
                 else:
                     year = int(match.group("year"))
                     optional_month = match.group("month")
diff --git a/tests/test_item_search.py b/tests/test_item_search.py
index 432bb475..bc7b177d 100644
--- a/tests/test_item_search.py
+++ b/tests/test_item_search.py
@@ -155,30 +155,34 @@ def test_mixed_simple_date_strings(self):
         search = ItemSearch(url=SEARCH_URL, datetime="2019/2020-06-10")
         assert search._parameters['datetime'] == "2019-01-01T00:00:00Z/2020-06-10T23:59:59Z"
 
+    def test_time(self):
+        search = ItemSearch(url=SEARCH_URL, datetime="2019-01-01T00:00:00Z/2019-01-01T00:12:00")
+        assert search._parameters['datetime'] == "2019-01-01T00:00:00Z/2019-01-01T00:12:00Z"
+
     def test_many_datetimes(self):
         datetimes = [
-            "1985-04-12T23:20:50.52Z"
-            "1996-12-19T16:39:57-08:00"
-            "1990-12-31T23:59:60Z"
-            "1990-12-31T15:59:60-08:00"
-            "1937-01-01T12:00:27.87+01:00"
-            "1985-04-12T23:20:50.52Z"
-            "1937-01-01T12:00:27.8710+01:00"
-            "1937-01-01T12:00:27.8+01:00"
-            "1937-01-01T12:00:27.8Z"
-            "1985-04-12t23:20:50.5202020z"
-            "2020-07-23T00:00:00Z"
-            "2020-07-23T00:00:00.0Z"
-            "2020-07-23T00:00:00.01Z"
-            "2020-07-23T00:00:00.012Z"
-            "2020-07-23T00:00:00.0123Z"
-            "2020-07-23T00:00:00.01234Z"
-            "2020-07-23T00:00:00.012345Z"
-            "2020-07-23T00:00:00.000Z"
-            "2020-07-23T00:00:00.000+03:00"
-            "2020-07-23T00:00:00+03:00"
-            "2020-07-23T00:00:00.000+03:00"
-            "2020-07-23T00:00:00.000z"
+            "1985-04-12T23:20:50.52Z",
+            "1996-12-19T16:39:57-08:00",
+            "1990-12-31T23:59:60Z",
+            "1990-12-31T15:59:60-08:00",
+            "1937-01-01T12:00:27.87+01:00",
+            "1985-04-12T23:20:50.52Z",
+            "1937-01-01T12:00:27.8710+01:00",
+            "1937-01-01T12:00:27.8+01:00",
+            "1937-01-01T12:00:27.8Z",
+            "1985-04-12t23:20:50.5202020z",
+            "2020-07-23T00:00:00Z",
+            "2020-07-23T00:00:00.0Z",
+            "2020-07-23T00:00:00.01Z",
+            "2020-07-23T00:00:00.012Z",
+            "2020-07-23T00:00:00.0123Z",
+            "2020-07-23T00:00:00.01234Z",
+            "2020-07-23T00:00:00.012345Z",
+            "2020-07-23T00:00:00.000Z",
+            "2020-07-23T00:00:00.000+03:00",
+            "2020-07-23T00:00:00+03:00",
+            "2020-07-23T00:00:00.000+03:00",
+            "2020-07-23T00:00:00.000z",
         ]
         for date_time in datetimes:
             ItemSearch(url=SEARCH_URL, datetime=date_time)