Core_Email::_set_date

protected function _set_date()
{
    $timezone = date('Z');
    $operator = ($timezone[0] === '-') ? '-' : '+';
    $timezone = abs($timezone);
    $timezone = floor($timezone/3600) * 100 + ($timezone % 3600) / 60;

    return sprintf('%s %s%04d', date('D, j M Y H:i:s'), $operator, $timezone);
}