use support.run_with_tz

Author: ynir3

Doc/library/smtplib.rst

@@ -519,7 +519,7 @@ An :class:`SMTP` instance has the following methods:
    is no way to unambiguously detect the most recent set of
    :mailheader:`Resent-` headers.
 
-   .. versionchanged:: 3.15
+   .. versionchanged:: next
       Support to add :mailheader:`Date` header to the message if one does not exist.
 
    ``send_message`` serializes *msg* using

Lib/test/test_smtplib.py

@@ -26,6 +26,7 @@
 from unittest.mock import Mock
 
 
+
 support.requires_working_socket(module=True)
 
 HOST = socket_helper.HOST
@@ -1393,11 +1394,6 @@ class SMTPUTF8SimTests(unittest.TestCase):
 
     maxDiff = None
 
-    def _is_local(self, date: str) -> bool:
-        dt = email.utils.parsedate_to_datetime(date)
-        local_offset = dt.now().astimezone().utcoffset()
-        return dt.utcoffset() == local_offset
-
     def setUp(self):
         self.thread_key = threading_helper.threading_setup()
         self.real_getfqdn = socket.getfqdn
@@ -1472,6 +1468,7 @@ def test_send_unicode_with_SMTPUTF8_via_low_level_API(self):
         self.assertIn('SMTPUTF8', self.serv.last_mail_options)
         self.assertEqual(self.serv.last_rcpt_options, [])
 
+    @support.run_with_tz('UTC-2')
     def test_send_message_uses_smtputf8_if_addrs_non_ascii(self):
         msg = EmailMessage()
         msg['From'] = "Páolo <fő[email protected]>"
@@ -1491,7 +1488,10 @@ def test_send_message_uses_smtputf8_if_addrs_non_ascii(self):
         last_message = self.serv.last_message.decode()
         date = email.message_from_string(last_message)['Date']
         # asserts RFC 5322 section 3.3 4th Paragraph
-        self.assertTrue(self._is_local(date))
+        self.assertEqual(
+            email.utils.parsedate_to_datetime(date).tzname(),
+            "UTC+02:00"
+        )
 
         expected = textwrap.dedent("""\
             From: Páolo <fő[email protected]>

Misc/NEWS.d/next/Library/2025-07-20-09-51-25.bpo-28879.SLDgcy.rst

@@ -1,2 +1,2 @@
 Fix ``smtplib.send_message`` to add a ``Date`` header if it is missing as
-per RFC5322.
+per :rfc:`5322`.