diff --git a/firebase_admin/_messaging_encoder.py b/firebase_admin/_messaging_encoder.py
index c4da53f0d..48a3dd3cd 100644
--- a/firebase_admin/_messaging_encoder.py
+++ b/firebase_admin/_messaging_encoder.py
@@ -324,7 +324,11 @@ def encode_android_notification(cls, notification):
 
         event_time = result.get('event_time')
         if event_time:
-            result['event_time'] = str(event_time.isoformat()) + 'Z'
+            # if the datetime instance is not naive (tzinfo is present), convert to UTC
+            # otherwise (tzinfo is None) assume the datetime instance is already in UTC
+            if event_time.tzinfo is not None:
+                event_time = event_time.astimezone(datetime.timezone.utc)
+            result['event_time'] = event_time.strftime('%Y-%m-%dT%H:%M:%S.%fZ')
 
         priority = result.get('notification_priority')
         if priority:
diff --git a/firebase_admin/_messaging_utils.py b/firebase_admin/_messaging_utils.py
index 3a1943c04..d25ba5520 100644
--- a/firebase_admin/_messaging_utils.py
+++ b/firebase_admin/_messaging_utils.py
@@ -98,7 +98,8 @@ class AndroidNotification:
             the user clicks it (optional).
         event_timestamp: For notifications that inform users about events with an absolute time
             reference, sets the time that the event in the notification occurred as a
-            ``datetime.datetime`` instance. Notifications in the panel are sorted by this time
+            ``datetime.datetime`` instance. If the ``datetime.datetime`` instance is naive, it
+            defaults to be in the UTC timezone. Notifications in the panel are sorted by this time
             (optional).
         local_only: Sets whether or not this notification is relevant only to the current device.
             Some notifications can be bridged to other devices for remote display, such as a Wear OS
diff --git a/tests/test_messaging.py b/tests/test_messaging.py
index f8be4cd67..f2ef47cf8 100644
--- a/tests/test_messaging.py
+++ b/tests/test_messaging.py
@@ -547,7 +547,10 @@ def test_android_notification(self):
                     click_action='ca', title_loc_key='tlk', body_loc_key='blk',
                     title_loc_args=['t1', 't2'], body_loc_args=['b1', 'b2'], channel_id='c',
                     ticker='ticker', sticky=True,
-                    event_timestamp=datetime.datetime(2019, 10, 20, 15, 12, 23, 123),
+                    event_timestamp=datetime.datetime(
+                        2019, 10, 20, 15, 12, 23, 123,
+                        tzinfo=datetime.timezone(datetime.timedelta(hours=-5))
+                    ),
                     local_only=False,
                     priority='high', vibrate_timings_millis=[100, 50, 250],
                     default_vibrate_timings=False, default_sound=True,
@@ -577,7 +580,7 @@ def test_android_notification(self):
                     'channel_id': 'c',
                     'ticker': 'ticker',
                     'sticky': True,
-                    'event_time': '2019-10-20T15:12:23.000123Z',
+                    'event_time': '2019-10-20T20:12:23.000123Z',
                     'local_only': False,
                     'notification_priority': 'PRIORITY_HIGH',
                     'vibrate_timings': ['0.100000000s', '0.050000000s', '0.250000000s'],
@@ -601,6 +604,28 @@ def test_android_notification(self):
         }
         check_encoding(msg, expected)
 
+    def test_android_notification_naive_event_timestamp(self):
+        event_time = datetime.datetime.now()
+        msg = messaging.Message(
+            topic='topic',
+            android=messaging.AndroidConfig(
+                notification=messaging.AndroidNotification(
+                    title='t',
+                    event_timestamp=event_time,
+                )
+            )
+        )
+        expected = {
+            'topic': 'topic',
+            'android': {
+                'notification': {
+                    'title': 't',
+                    'event_time': event_time.strftime('%Y-%m-%dT%H:%M:%S.%fZ')
+                },
+            },
+        }
+        check_encoding(msg, expected)
+
 
 class TestLightSettingsEncoder: