diff --git a/slack_sdk/oauth/installation_store/internals.py b/slack_sdk/oauth/installation_store/internals.py
index 52eeeb98b..aa344d664 100644
--- a/slack_sdk/oauth/installation_store/internals.py
+++ b/slack_sdk/oauth/installation_store/internals.py
@@ -1,4 +1,4 @@
-from datetime import datetime
+from datetime import datetime, timezone
 from typing import Type, TypeVar, Union
 
 
@@ -28,6 +28,9 @@ def _timestamp_to_type(ts: Union[TimestampType, datetime, str], target_type: Typ
         # see https://github.com/google/pytype/issues/1012
 
     elif isinstance(ts, datetime):
+        if ts.tzinfo is None:
+            # naive datetime values (e.g., loaded from a database) are stored in UTC
+            ts = ts.replace(tzinfo=timezone.utc)
         result = target_type(ts.timestamp())
     elif isinstance(ts, str):
         try:
diff --git a/tests/slack_sdk/oauth/installation_store/test_internals.py b/tests/slack_sdk/oauth/installation_store/test_internals.py
index 74cb935fc..a34dc617c 100644
--- a/tests/slack_sdk/oauth/installation_store/test_internals.py
+++ b/tests/slack_sdk/oauth/installation_store/test_internals.py
@@ -1,6 +1,9 @@
+import os
 import sys
+import time
 import unittest
 from datetime import datetime, timezone
+from unittest import mock
 
 import pytest
 
@@ -38,6 +41,16 @@ def test_timestamp_to_type(ts, target_type, expected_result):
     assert result == expected_result
 
 
+@pytest.mark.skipif(not hasattr(time, "tzset"), reason="time.tzset is not available on this platform")
+def test_timestamp_to_type_naive_datetime_is_utc():
+    try:
+        with mock.patch.dict(os.environ, {"TZ": "America/New_York"}):
+            time.tzset()
+            assert _timestamp_to_type(datetime(2023, 11, 28, 22, 9, 7), int) == 1701209347
+    finally:
+        time.tzset()
+
+
 def test_timestamp_to_type_invalid_str():
     match = "Invalid isoformat string"
     with pytest.raises(ValueError, match=match):