diff --git a/aries_cloudagent/messaging/tests/test_util.py b/aries_cloudagent/messaging/tests/test_util.py
index 23614b511e..c1a7e7c82b 100644
--- a/aries_cloudagent/messaging/tests/test_util.py
+++ b/aries_cloudagent/messaging/tests/test_util.py
@@ -55,7 +55,7 @@ def test_format(self):
         tests = {
             datetime(
                 2019, 5, 17, 20, 51, 19, 519437, tzinfo=timezone.utc
-            ): "2019-05-17 20:51:19.519437Z",
+            ): "2019-05-17T20:51:19.519437Z",
             now: now,
         }
         for datetime_val, expected in tests.items():
diff --git a/aries_cloudagent/messaging/util.py b/aries_cloudagent/messaging/util.py
index d100d4adab..a5e28b15a1 100644
--- a/aries_cloudagent/messaging/util.py
+++ b/aries_cloudagent/messaging/util.py
@@ -21,7 +21,7 @@ def datetime_to_str(dt: Union[str, datetime]) -> str:
         dt: May be a string or datetime to allow automatic conversion
     """
     if isinstance(dt, datetime):
-        dt = dt.replace(tzinfo=timezone.utc).isoformat(" ").replace("+00:00", "Z")
+        dt = dt.replace(tzinfo=timezone.utc).isoformat().replace("+00:00", "Z")
     return dt