iso8601_to_datetime( string $date_string, string $timezone = 'user' )

function iso8601_to_datetime( $date_string, $timezone = 'user' ) {
	$timezone = strtolower($timezone);

	if ($timezone == 'gmt') {

		preg_match('#([0-9]{4})([0-9]{2})([0-9]{2})T([0-9]{2}):([0-9]{2}):([0-9]{2})(Z|[\+|\-][0-9]{2,4}){0,1}#', $date_string, $date_bits);

		if (!empty($date_bits[7])) { // we have a timezone, so let's compute an offset
			$offset = iso8601_timezone_to_offset($date_bits[7]);
		} else { // we don't have a timezone, so we assume user local timezone (not server's!)
			$offset = HOUR_IN_SECONDS * get_option('gmt_offset');
		}

		$timestamp = gmmktime($date_bits[4], $date_bits[5], $date_bits[6], $date_bits[2], $date_bits[3], $date_bits[1]);
		$timestamp -= $offset;

		return gmdate('Y-m-d H:i:s', $timestamp);

	} elseif ($timezone == 'user') {
		return preg_replace('#([0-9]{4})([0-9]{2})([0-9]{2})T([0-9]{2}):([0-9]{2}):([0-9]{2})(Z|[\+|\-][0-9]{2,4}){0,1}#', '$1-$2-$3 $4:$5:$6', $date_string);
	}
}