session expiration datetime is UTC timezone-aware

CHANGES.rst

@@ -36,6 +36,8 @@ Unreleased
 -   It is no longer required to decorate custom CLI commands on
     ``app.cli`` or ``blueprint.cli`` with ``@with_appcontext``, an app
     context will already be active at that point. :issue:`2410`
+-   ``SessionInterface.get_expiration_time`` uses a timezone-aware
+    value. :pr:`4645`
 
 
 Version 2.1.3

src/flask/sessions.py

@@ -3,6 +3,7 @@ import typing as t
 import warnings
 from collections.abc import MutableMapping
 from datetime import datetime
+from datetime import timezone
 
 from itsdangerous import BadSignature
 from itsdangerous import URLSafeTimedSerializer
@@ -277,7 +278,7 @@ class SessionInterface:
         lifetime configured on the application.
         """
         if session.permanent:
-            return datetime.utcnow() + app.permanent_session_lifetime
+            return datetime.now(timezone.utc) + app.permanent_session_lifetime
         return None
 
     def should_set_cookie(self, app: "Flask", session: SessionMixin) -> bool:

tests/test_basic.py

@@ -5,6 +5,7 @@ import uuid
 import warnings
 import weakref
 from datetime import datetime
+from datetime import timezone
 from platform import python_implementation
 from threading import Thread
 
@@ -436,7 +437,7 @@ def test_session_expiration(app, client):
     assert "set-cookie" in rv.headers
     match = re.search(r"(?i)\bexpires=([^;]+)", rv.headers["set-cookie"])
     expires = parse_date(match.group())
-    expected = datetime.utcnow() + app.permanent_session_lifetime
+    expected = datetime.now(timezone.utc) + app.permanent_session_lifetime
     assert expires.year == expected.year
     assert expires.month == expected.month
     assert expires.day == expected.day
@@ -466,7 +467,7 @@ def test_session_stored_last(app, client):
 
 
 def test_session_special_types(app, client):
-    now = datetime.utcnow().replace(microsecond=0)
+    now = datetime.now(timezone.utc).replace(microsecond=0)
     the_uuid = uuid.uuid4()
 
     @app.route("/")