#run a cron once a day at 3am
0 3 * * * +---------------- minute (0 - 59)
| +------------- hour (0 - 23)
| | +---------- day of month (1 - 31)
| | | +------- month (1 - 12)
| | | | +---- day of week (0 - 6) (Sunday=0 or 7)
| | | | |
* * * * * command to be executed
30 2 * * * /your/command
# ^ ^
# | hour
# minute