Graphically, the cron syntax for Quarz is (source):
+-------------------- second (0 - 59)
| +----------------- minute (0 - 59)
| | +-------------- hour (0 - 23)
| | | +----------- day of month (1 - 31)
| | | | +-------- month (1 - 12)
| | | | | +----- day of week (0 - 6) (Sunday=0 or 7)
| | | | | | +-- year [optional]
| | | | | | |
* * * * * * * command to be executed
So if you want to run a command every 30 minutes you can say either of these:
0 0/30 * * * * ?
0 0,30 * * * * ?
You can check crontab expressions using either of these: