POSIX/t/time.t: Add tests for DST changes

Author: khwilliamson

ext/POSIX/t/time.t

@@ -9,7 +9,7 @@ use strict;
 
 use Config;
 use POSIX;
-use Test::More tests => 31;
+use Test::More tests => 49;
 
 # For the first go to UTC to avoid DST issues around the world when testing.  SUS3 says that
 # null should get you UTC, but some environments want the explicit names.
@@ -226,3 +226,66 @@ SKIP: {
     is(strftime(undef, CORE::localtime), '', "strftime() works if format is undef");
     like($warnings, qr/^Use of uninitialized value in subroutine entry /, "strftime(undef, ...) produces expected warning");
 }
+
+SKIP: {   # GH #23878; test that dst fall back works properly
+    my $skip_count = 9;
+    skip "No mktime()", $skip_count if $Config{d_mktime} ne 'define';
+    my $locale = "Europe/Paris";
+    $ENV{TZ} = $locale;
+    my $t = 1761436800;     # an hour before time should have changed
+
+    # The time in the first test case is such that UTC gives a different day.
+    # If the locale above is unknown, libc is supposed to use UTC; so that's
+    # how we check if the system knows about the rules for Paris; which on
+    # some systems differ from plain CET-1CEST.
+    skip "'$locale' not understood", $skip_count if
+                POSIX::strftime("%F %T%z", localtime($t - 1)) !~ /2025-10-26/;
+
+    # On Windows, the documentation says that it won't understand what our
+    # $locale is set to, but instead of falling back to UTC, it uses the
+    # system timezone, or U.S.  Pacific time.  The trick above therefore
+    # doesn't work, so just skip this batch of tests.
+    skip "'$locale' not understood", $skip_count if $^O eq "MSWin32";
+
+    my @fall = (
+                 [   -1, "2025-10-26 01:59:59+0200", "Chg -1 hr, 1 sec" ],
+                 [    0, "2025-10-26 02:00:00+0200", "Chg -1 hr, 0 sec" ],
+                 [    1, "2025-10-26 02:00:01+0200", "Chg -59 min, 59 sec" ],
+                 [ 3599, "2025-10-26 02:59:59+0200", "Chg -1 sec" ],
+                 [ 3600, "2025-10-26 02:00:00+0100", "At Paris DST fallback" ],
+                 [ 3601, "2025-10-26 02:00:01+0100", "Chg +1 sec" ],
+                 [ 7199, "2025-10-26 02:59:59+0100", "Chg +1 hr, 59m, 59s" ],
+                 [ 7200, "2025-10-26 03:00:00+0100", "Chg +1 hr" ],
+                 [ 7201, "2025-10-26 03:00:01+0100", "Chg +1 hr, 1 sec" ],
+               );
+    for (my $i = 0; $i < @fall; $i++) {
+        is(POSIX::strftime("%F %T%z", localtime $t + $fall[$i][0]),
+           $fall[$i][1], $fall[$i][2]);
+    }
+}
+
+SKIP: {   # GH #23878: test that dst spring forward works properly; use a
+          # locale that MS narcissism should be able to handle
+    my $skip_count = 9;
+    skip "No mktime()", $skip_count if $Config{d_mktime} ne 'define';
+    my $locale = "PST8PDT";
+    $ENV{TZ} = $locale;
+    my $t = 1741510800;     # an hour before time should have changed
+
+    my @spring = (
+                  [   -1, "2025-03-09 00:59:59-0800", "Chg -1 hr, 1 sec" ],
+                  [    0, "2025-03-09 01:00:00-0800", "Chg -1 hr, 0 sec" ],
+                  [    1, "2025-03-09 01:00:01-0800", "Chg -59 min, 59 sec" ],
+                  [ 3599, "2025-03-09 01:59:59-0800", "Chg -1 sec" ],
+                  [ 3600, "2025-03-09 03:00:00-0700",
+                                            "At Redmond DST spring forward" ],
+                  [ 3601, "2025-03-09 03:00:01-0700", "Chg +1 sec" ],
+                  [ 7199, "2025-03-09 03:59:59-0700", "Chg +1 hr, 59m, 59s" ],
+                  [ 7200, "2025-03-09 04:00:00-0700", "Chg +1 hr" ],
+                  [ 7201, "2025-03-09 04:00:01-0700", "Chg +1 hr, 1 sec" ],
+               );
+    for (my $i = 0; $i < @spring; $i++) {
+        is(POSIX::strftime("%F %T%z", localtime $t + $spring[$i][0]),
+           $spring[$i][1], $spring[$i][2]);
+    }
+}