diff --git a/src/aws_durable_execution_sdk_python_testing/web/serialization.py b/src/aws_durable_execution_sdk_python_testing/web/serialization.py
index 93ae247..c5ab215 100644
--- a/src/aws_durable_execution_sdk_python_testing/web/serialization.py
+++ b/src/aws_durable_execution_sdk_python_testing/web/serialization.py
@@ -73,10 +73,10 @@ def to_bytes(self, data: Any) -> bytes:
                 f"Failed to serialize data to JSON: {str(e)}"
             )
 
-    def _default_handler(self, obj: Any) -> str:
+    def _default_handler(self, obj: Any) -> float:
         """Handle non-permitive objects."""
         if isinstance(obj, datetime):
-            return obj.isoformat()
+            return obj.timestamp()
         # Raise TypeError for unsupported types
         raise TypeError(f"Object of type {type(obj).__name__} is not JSON serializable")
 
diff --git a/tests/web/serialization_test.py b/tests/web/serialization_test.py
index da518ae..c34c8c4 100644
--- a/tests/web/serialization_test.py
+++ b/tests/web/serialization_test.py
@@ -6,7 +6,7 @@
 
 import pytest
 import json
-from datetime import datetime
+from datetime import datetime, timezone
 
 from aws_durable_execution_sdk_python_testing.exceptions import (
     InvalidParameterValueException,
@@ -404,23 +404,23 @@ def test_serialize_simple_dict():
 def test_serialize_datetime():
     """Test serialization of datetime objects."""
     serializer = JSONSerializer()
-    now = datetime(2025, 11, 5, 16, 30, 9, 895000)
+    now = datetime(2025, 11, 5, 16, 30, 9, 895000, tzinfo=timezone.utc)
     data = {"timestamp": now}
 
     result = serializer.to_bytes(data)
-    expected = b'{"timestamp":"2025-11-05T16:30:09.895000"}'
+    expected = b'{"timestamp":1762360209.895}'
 
     assert result == expected
     assert isinstance(result, bytes)
 
     deserialized = json.loads(result.decode("utf-8"))
-    assert deserialized["timestamp"] == "2025-11-05T16:30:09.895000"
+    assert deserialized["timestamp"] == now.timestamp()
 
 
 def test_serialize_nested_datetime():
     """Test serialization of nested structures with datetime."""
     serializer = JSONSerializer()
-    now = datetime(2025, 11, 5, 16, 30, 9)
+    now = datetime(2025, 11, 5, 16, 30, 9, tzinfo=timezone.utc)
     data = {
         "event": "user_login",
         "timestamp": now,
@@ -430,22 +430,22 @@ def test_serialize_nested_datetime():
     result = serializer.to_bytes(data)
     expected = (
         b'{"event":"user_login",'
-        b'"timestamp":"2025-11-05T16:30:09",'
-        b'"metadata":{"created_at":"2025-11-05T16:30:09",'
-        b'"updated_at":"2025-11-05T16:30:09"}}'
+        b'"timestamp":1762360209.0,'
+        b'"metadata":{"created_at":1762360209.0,'
+        b'"updated_at":1762360209.0}}'
     )
 
     assert result == expected
 
     deserialized = json.loads(result.decode("utf-8"))
-    assert deserialized["timestamp"] == now.isoformat()
-    assert deserialized["metadata"]["created_at"] == now.isoformat()
+    assert deserialized["timestamp"] == now.timestamp()
+    assert deserialized["metadata"]["created_at"] == now.timestamp()
 
 
 def test_serialize_list_with_datetime():
     """Test serialization of list containing datetime."""
     serializer = JSONSerializer()
-    now = datetime(2025, 11, 5, 16, 30, 9)
+    now = datetime(2025, 11, 5, 16, 30, 9, tzinfo=timezone.utc)
     data = {
         "events": [{"time": now, "action": "login"}, {"time": now, "action": "logout"}]
     }
@@ -453,22 +453,22 @@ def test_serialize_list_with_datetime():
     result = serializer.to_bytes(data)
     expected = (
         b'{"events":['
-        b'{"time":"2025-11-05T16:30:09","action":"login"},'
-        b'{"time":"2025-11-05T16:30:09","action":"logout"}'
+        b'{"time":1762360209.0,"action":"login"},'
+        b'{"time":1762360209.0,"action":"logout"}'
         b"]}"
     )
 
     assert result == expected
 
     deserialized = json.loads(result.decode("utf-8"))
-    assert deserialized["events"][0]["time"] == now.isoformat()
-    assert deserialized["events"][1]["time"] == now.isoformat()
+    assert deserialized["events"][0]["time"] == now.timestamp()
+    assert deserialized["events"][1]["time"] == now.timestamp()
 
 
 def test_serialize_mixed_types():
     """Test serialization of mixed data types."""
     serializer = JSONSerializer()
-    now = datetime(2025, 11, 5, 16, 30, 9)
+    now = datetime(2025, 11, 5, 16, 30, 9, tzinfo=timezone.utc)
     data = {
         "string": "test",
         "number": 42,
@@ -487,7 +487,7 @@ def test_serialize_mixed_types():
         b'"boolean":true,'
         b'"null":null,'
         b'"list":[1,2,3],'
-        b'"datetime":"2025-11-05T16:30:09"}'
+        b'"datetime":1762360209.0}'
     )
 
     assert result == expected
@@ -499,7 +499,7 @@ def test_serialize_mixed_types():
     assert deserialized["boolean"] is True
     assert deserialized["null"] is None
     assert deserialized["list"] == [1, 2, 3]
-    assert deserialized["datetime"] == now.isoformat()
+    assert deserialized["datetime"] == now.timestamp()
 
 
 def test_serialize_returns_bytes():
@@ -546,11 +546,11 @@ def test_serialize_circular_reference_raises_exception():
 def test_serialize_datetime_with_microseconds():
     """Test serialization of datetime with microseconds."""
     serializer = JSONSerializer()
-    now = datetime(2025, 11, 5, 16, 30, 9, 123456)
+    now = datetime(2025, 11, 5, 16, 30, 9, 123456, tzinfo=timezone.utc)
     data = {"timestamp": now}
 
     result = serializer.to_bytes(data)
-    expected = b'{"timestamp":"2025-11-05T16:30:09.123456"}'
+    expected = b'{"timestamp":1762360209.123456}'
 
     assert result == expected
 
@@ -558,11 +558,11 @@ def test_serialize_datetime_with_microseconds():
 def test_serialize_datetime_without_microseconds():
     """Test serialization of datetime without microseconds."""
     serializer = JSONSerializer()
-    now = datetime(2025, 11, 5, 16, 30, 9)
+    now = datetime(2025, 11, 5, 16, 30, 9, tzinfo=timezone.utc)
     data = {"timestamp": now}
 
     result = serializer.to_bytes(data)
-    expected = b'{"timestamp":"2025-11-05T16:30:09"}'
+    expected = b'{"timestamp":1762360209.0}'
 
     assert result == expected
 
@@ -570,11 +570,11 @@ def test_serialize_datetime_without_microseconds():
 def test_serialize_multiple_datetimes():
     """Test multiple datetime objects."""
     serializer = JSONSerializer()
-    dt1 = datetime(2025, 1, 1, 0, 0, 0)
-    dt2 = datetime(2025, 12, 31, 23, 59, 59)
+    dt1 = datetime(2025, 1, 1, 0, 0, 0, tzinfo=timezone.utc)
+    dt2 = datetime(2025, 12, 31, 23, 59, 59, tzinfo=timezone.utc)
 
     data = {"start": dt1, "end": dt2}
     result = serializer.to_bytes(data)
-    expected = b'{"start":"2025-01-01T00:00:00",' b'"end":"2025-12-31T23:59:59"}'
+    expected = b'{"start":1735689600.0,"end":1767225599.0}'
 
     assert result == expected