Replace strptime for Windows compatibility

[BitPoet]

The strptime function isn't implemented in PHP on Windows, so the plugin fails to work there. A platform independent replacement would be:

    static function dateToTimestamp($dateString) {
        $a = date_create_from_format('Y-m-d H:i:s', $dateString);
        return $a->getTimestamp();
    }

[deanmoses]

Thanks! Will fix.