fix: strftime() sets epoch index for %G and %s format codes

strftime() never assigned $me->[9] (the epoch timestamp), so format
codes that read it — %G (GPS week) and %s (epoch seconds) — produced
wrong results. %G returned -522 instead of the correct GPS week number
because the uninitialized value was treated as zero.

Set $me->[9] in both the timezone and non-timezone code paths, matching
what time2str() already does.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Koan-Bot

lib/Date/Format/Generic.pm

@@ -56,11 +56,13 @@ sub strftime
    $epoch = timelocal(@{$time}[0..5]);
 
    @$me = gmtime($epoch + tz_offset($tzname));
+   $me->[9] = $epoch;
   }
  else
   {
    @$me = @$time;
-   undef $epoch;
+   $epoch = timelocal(@$me[0..5]);
+   $me->[9] = $epoch;
   }
 
  _subs($me,$fmt);

t/strftime-epoch.t

@@ -0,0 +1,48 @@
+use strict;
+use warnings;
+use Test::More;
+use Date::Format qw(time2str strftime);
+use Time::Local qw(timegm);
+
+# strftime() was missing $me->[9] (epoch timestamp), causing format codes
+# that depend on it (%G, %s) to produce wrong results.
+
+my $t = timegm(0, 0, 12, 28, 3, 124);  # 2024-04-28 12:00:00 UTC
+my @gmt = gmtime($t);
+
+# %G (GPS week) - was returning -522 instead of 2312
+is(time2str("%G", $t, "UTC"), "2312",
+    "time2str %G gives correct GPS week");
+is(strftime("%G", @gmt, "UTC"), "2312",
+    "strftime %G with timezone gives correct GPS week");
+
+# %s (epoch seconds) - strftime with localtime should match time2str
+my @lt = localtime($t);
+is(time2str("%s", $t), strftime("%s", @lt),
+    "strftime %s (localtime, no tz) matches time2str %s");
+
+# GPS epoch boundary via strftime
+my $gps_epoch = 315964800;
+my @gps_gmt = gmtime($gps_epoch);
+is(strftime("%G", @gps_gmt, "UTC"), "0",
+    "strftime %G at GPS epoch is week 0");
+
+my @week1_gmt = gmtime($gps_epoch + 7 * 86400);
+is(strftime("%G", @week1_gmt, "UTC"), "1",
+    "strftime %G one week after GPS epoch is week 1");
+
+# Verify strftime and time2str agree on %G for several dates
+for my $pair (
+    [timegm(0, 0, 0, 6, 0, 80),  "0",    "GPS epoch"],
+    [timegm(0, 0, 0, 13, 0, 80), "1",    "start of week 1"],
+    [936709362,                    "1026", "Sep 7, 1999"],
+) {
+    my ($epoch, $expected, $label) = @$pair;
+    my @g = gmtime($epoch);
+    is(strftime("%G", @g, "UTC"), $expected,
+        "strftime %G: $label");
+    is(time2str("%G", $epoch, "UTC"), $expected,
+        "time2str %G: $label (cross-check)");
+}
+
+done_testing;