diff --git a/mmif/serialize/mmif.py b/mmif/serialize/mmif.py
index 68478c7f..245c96aa 100644
--- a/mmif/serialize/mmif.py
+++ b/mmif/serialize/mmif.py
@@ -14,7 +14,7 @@
 import math
 import warnings
 from collections import defaultdict
-from datetime import datetime
+from datetime import datetime, timezone
 from typing import Any, List, Union, Optional, Dict, cast, Iterator
 
 import jsonschema.validators
@@ -433,7 +433,7 @@ def new_view(self) -> View:
         """
         new_view = View()
         new_view.id = self.new_view_id()
-        new_view.metadata.timestamp = datetime.now()
+        new_view.metadata.timestamp = datetime.now(timezone.utc)
         self.add_view(new_view)
         return new_view
 
diff --git a/mmif/serialize/model.py b/mmif/serialize/model.py
index 1bec7b29..95fdc28c 100644
--- a/mmif/serialize/model.py
+++ b/mmif/serialize/model.py
@@ -402,7 +402,10 @@ def default(self, obj: 'MmifObject'):
         if hasattr(obj, '_serialize'):
             return obj._serialize()
         elif hasattr(obj, 'isoformat'):         # for datetime objects
-            return obj.isoformat()
+            s = obj.isoformat()
+            if s.endswith('+00:00'):
+                s = s[:-6] + 'Z'
+            return s
         elif hasattr(obj, '__str__'):
             return str(obj)
         else:
diff --git a/tests/test_serialize.py b/tests/test_serialize.py
index 9e857a00..f5b0846f 100644
--- a/tests/test_serialize.py
+++ b/tests/test_serialize.py
@@ -608,6 +608,25 @@ def test_get_label(self):
             a = v.new_annotation(AnnotationTypes.BoundingBox)
             _ = a._get_label()
 
+    def test_timestamp_uses_utc_with_z_suffix(self):
+        """Test that timestamps are in UTC with 'Z' suffix to avoid ambiguity"""
+        from datetime import timezone
+        mmif_obj = Mmif(validate=False)
+
+        new_view = mmif_obj.new_view()
+        new_view.metadata.app = "http://test.app"
+
+        # Verify the timestamp is timezone-aware and uses UTC
+        self.assertIsNotNone(new_view.metadata.timestamp)
+        self.assertIsNotNone(new_view.metadata.timestamp.tzinfo)
+        self.assertEqual(new_view.metadata.timestamp.tzinfo, timezone.utc)
+
+        # Verify serialization uses 'Z' suffix instead of '+00:00'
+        serialized = json.loads(mmif_obj.serialize())
+        ts = serialized['views'][0]['metadata']['timestamp']
+        self.assertTrue(ts.endswith('Z'))
+        self.assertNotIn('+00:00', ts)
+
     def test_get_anchor_point(self):
         mmif = Mmif(validate=False)
         v1 = mmif.new_view()