src/Peach/DT/HttpDateFormat.php
private function getFullYear($y)
    {
        $currentYear = intval(date("Y"));
        $century     = intval($currentYear / 100);
        $smallY      = $currentYear % 100;
        $resultC     = ($smallY < $y) ? $century - 1 : $century;
        return $resultC * 100 + $y;
    }
private function getFullYear($y)
    {
        $currentYear = intval(date("Y"));
        $century     = intval($currentYear / 100);
        $smallY      = $currentYear % 100;
        $resultC     = ($smallY < $y) ? $century - 1 : $century;
        return $resultC * 100 + $y;
    }
2 桁の年表示を 4 桁に変換します.
もしも引数の 2 桁年が, 現在の 2 桁年よりも大きい場合,
前世紀の年と判断します.

@param  int $y 変換対象の 2 桁年
@return int    4 桁の年