There’s a (?) logo on the crontab page (or should be, otherwise you’re using a very, very, very old commit). This page helps explain the use of crontab syntax to determine times to run actions.
It is important to note that while any other cron expression tutorial you read on the internet is valid and useful, this particular cron syntax offers an additional time gauge, with the added one being for “seconds” to give you added precision. For your case (which is not requiring precision), you would likely choose to make the “seconds” value “0”, because you’re interested in having restores every 30 minutes.
Here’s an example I generated from http://www.crontab-generator.org for every 30 minutes:
*/30 * * * *
This would translate into
0 */30 * * * *
in MineOS. You can use this second one–right now–for your 30 minute backups.