Address review comments for libc timezone globals

Rewrite test_strptime.test_timezone to check each tzname entry and treat
UTC/GMT as standard time, so zones with negative DST such as
Europe/Dublin pass.  Add a round-trip test with fixed EST5EDT rules.

Document the glibc tzset() early-return caveat in init_timezone().  In
get_dst_offset(), skip a normalized sample date instead of abandoning
the remaining months, and drop a dead tm_mon range check.

Expand the NEWS entry to cover the negative DST classification change,
daylight being nonzero for historical DST, and altzone sampling on
glibc and Windows.

Author: nascheme

Lib/test/test_strptime.py

@@ -474,18 +474,35 @@ def test_timezone(self):
         self.assertEqual(strp_output.tm_isdst, 0)
         strp_output = _strptime._strptime_time("GMT", "%Z")
         self.assertEqual(strp_output.tm_isdst, 0)
-        time_tuple = time.localtime()
-        strf_output = time.strftime("%Z")  #UTC does not have a timezone
-        strp_output = _strptime._strptime_time(strf_output, "%Z")
-        locale_time = _strptime.LocaleTime()
-        if time.tzname[0] != time.tzname[1] or not time.daylight:
-            self.assertTrue(strp_output[8] == time_tuple[8],
-                            "timezone check failed; '%s' -> %s != %s" %
-                             (strf_output, strp_output[8], time_tuple[8]))
-        else:
-            self.assertTrue(strp_output[8] == -1,
-                            "LocaleTime().timezone has duplicate values and "
-                             "time.daylight but timezone value not set to -1")
+        # %Z recognizes the module's timezone names.  UTC and GMT are always
+        # parsed as standard time, even in zones such as Europe/Dublin where
+        # libc reports GMT as (negative) DST.
+        for isdst, name in enumerate(time.tzname):
+            if not name or (isdst and not time.daylight):
+                continue
+            with self.subTest(name=name, isdst=isdst):
+                if name.lower() in ('utc', 'gmt'):
+                    expected = 0
+                elif time.daylight and time.tzname[0] == time.tzname[1]:
+                    expected = -1
+                else:
+                    expected = isdst
+                parsed = _strptime._strptime_time(name, '%Z')
+                self.assertEqual(parsed.tm_isdst, expected)
+
+    @support.run_with_tz('EST5EDT,M3.2.0,M11.1.0')
+    @skip_if_buggy_ucrt_strfptime
+    def test_timezone_roundtrip(self):
+        # Use stable, explicit rules when comparing strftime(), strptime(),
+        # and localtime(), including both standard time and DST.
+        for month, isdst in ((1, 0), (7, 1)):
+            with self.subTest(month=month):
+                t = time.mktime((2000, month, 1, 12, 0, 0, 0, 0, -1))
+                local = time.localtime(t)
+                self.assertEqual(local.tm_isdst, isdst)
+                name = time.strftime('%Z', local)
+                parsed = _strptime._strptime_time(name, '%Z')
+                self.assertEqual(parsed.tm_isdst, isdst)
 
     @unittest.skipUnless(
         hasattr(time, "tzset"), "time module has no attribute tzset"

Misc/NEWS.d/next/Library/2026-11-04-12-13-21.gh-issue-161194.GiNgY1.rst

@@ -3,5 +3,12 @@ On platforms where libc provides the ``timezone``, ``daylight`` and
 :data:`time.tzname` are now taken from libc instead of being guessed from
 samples of the local time in January and July.  This fixes stale values for
 zones whose rules change during the year, such as ``America/Vancouver`` in
-2026.  :data:`time.altzone` now equals :data:`time.timezone` for zones
-without DST when libc does not provide ``altzone``.
+2026.  Standard and DST are now classified by libc rather than by comparing
+offsets, so for zones with negative DST such as ``Europe/Dublin``,
+:data:`time.tzname` is now ``('IST', 'GMT')`` and :data:`time.timezone` is
+``-3600``, matching :attr:`~time.struct_time.tm_isdst`.  On some platforms
+:data:`time.daylight` may now be nonzero for zones that observed DST only in
+the past.  When libc does not provide ``altzone`` (including glibc and
+Windows), :data:`time.altzone` is now found by sampling the current year for
+a DST offset instead of assuming a one-hour shift, and equals
+:data:`time.timezone` for zones without DST.

Modules/timemodule.c

@@ -1898,9 +1898,6 @@ get_dst_offset(time_t now, long dst_offset)
         PyErr_Clear();
         return dst_offset;
     }
-    if (current.tm_mon < 0 || current.tm_mon > 11) {
-        return dst_offset;
-    }
 
     int found_dst = 0;
     for (int month = 0; month < 12; month++) {
@@ -1933,7 +1930,7 @@ get_dst_offset(time_t now, long dst_offset)
                 || (month < current.tm_mon && sample > now)
                 || (month > current.tm_mon && sample < now))
             {
-                break;
+                continue;
             }
         }
         if (p.tm_isdst > 0) {
@@ -1983,6 +1980,12 @@ init_timezone(PyObject *m)
 #ifdef _Py_HAVE_TIMEZONE_GLOBALS
     PyObject *otz0, *otz1;
 #if !defined(MS_WINDOWS) || defined(MS_WINDOWS_DESKTOP) || defined(MS_WINDOWS_SYSTEM)
+    /* glibc caveat: tzset() returns early if TZ is unchanged since the
+       last call, and localtime() updates tzname and daylight for the rules
+       in effect at the converted time.  So when time.tzset() is called
+       without changing TZ, the values read here reflect the most recent
+       localtime() call rather than a fresh read of the zone data.  There
+       is no portable way to force a re-read. */
     tzset();
 #endif
     ADD_INT("timezone", _Py_timezone);