iso8601_timezone_to_offset( string $timezone )

function iso8601_timezone_to_offset( $timezone ) {
	// $timezone is either 'Z' or '[+|-]hhmm'
	if ($timezone == 'Z') {
		$offset = 0;
	} else {
		$sign    = (substr($timezone, 0, 1) == '+') ? 1 : -1;
		$hours   = intval(substr($timezone, 1, 2));
		$minutes = intval(substr($timezone, 3, 4)) / 60;
		$offset  = $sign * HOUR_IN_SECONDS * ($hours + $minutes);
	}
	return $offset;
}