diff --git a/customerio/client_base.py b/customerio/client_base.py
index e333b08..db5b70a 100644
--- a/customerio/client_base.py
+++ b/customerio/client_base.py
@@ -123,6 +123,8 @@ def _sanitize_value(self, value):
         return value
 
     def _datetime_to_timestamp(self, dt):
+        if dt.tzinfo is not None:
+            return int(dt.astimezone(timezone.utc).timestamp())
         return int(dt.replace(tzinfo=timezone.utc).timestamp())
 
     def _stringify_list(self, customer_ids):
diff --git a/tests/test_customerio.py b/tests/test_customerio.py
index 1995e79..e175ad7 100644
--- a/tests/test_customerio.py
+++ b/tests/test_customerio.py
@@ -1,7 +1,7 @@
 import json
 import socket
 import unittest
-from datetime import datetime
+from datetime import datetime, timedelta, timezone
 from functools import partial
 
 import urllib3
@@ -562,12 +562,24 @@ def test_unsuppress_call(self):
             self.cio.unsuppress(None)
 
     def test_sanitize(self):
-        from datetime import timezone
-
         data_in = dict(dt=datetime(2009, 2, 13, 23, 31, 30, 0, timezone.utc))
         data_out = self.cio._sanitize(data_in)
         self.assertEqual(data_out, dict(dt=1234567890))
 
+    def test_sanitize_naive_datetime(self):
+        """Naive datetimes are assumed UTC (backward compatible)."""
+        data_in = dict(dt=datetime(2009, 2, 13, 23, 31, 30))
+        data_out = self.cio._sanitize(data_in)
+        self.assertEqual(data_out, dict(dt=1234567890))
+
+    def test_sanitize_aware_non_utc_datetime(self):
+        """Tz-aware non-UTC datetimes are converted, not silently replaced."""
+        # 2009-02-13 18:31:30 at UTC-5 is 2009-02-13 23:31:30 UTC
+        tz_minus_5 = timezone(timedelta(hours=-5))
+        data_in = dict(dt=datetime(2009, 2, 13, 18, 31, 30, 0, tz_minus_5))
+        data_out = self.cio._sanitize(data_in)
+        self.assertEqual(data_out, dict(dt=1234567890))
+
     def test_ids_are_encoded_in_url(self):
         self.cio.http.hooks = dict(
             response=partial(