src/Peach/Http/Header/CookieExpiresFormat.php
private function handleFormat(Timestamp $t)
    {
        $wdy = $this->formatWeekday($t->getDay());
        $dd  = str_pad($t->get("date"), 2, "0", STR_PAD_LEFT);
        $mon = $this->formatMonth($t->get("month"));
        $yy  = str_pad($t->get("year"), 4, "0", STR_PAD_LEFT);
        $hms = $t->formatTime();
        return "{$wdy}, {$dd}-{$mon}-{$yy} {$hms} GMT";
    }
private function handleFormat(Timestamp $t)
    {
        $wdy = $this->formatWeekday($t->getDay());
        $dd  = str_pad($t->get("date"), 2, "0", STR_PAD_LEFT);
        $mon = $this->formatMonth($t->get("month"));
        $yy  = str_pad($t->get("year"), 4, "0", STR_PAD_LEFT);
        $hms = $t->formatTime();
        return "{$wdy}, {$dd}-{$mon}-{$yy} {$hms} GMT";
    }
format() の具体的な実装です. 引数の時刻は GMT に変換されているものとします.

@param  Timestamp $t 変換対象の時刻 (GMT)
@return string 変換結果