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Cron is a useful scheduling system.

* * * * *
- - - - -
| | | | |
| | | | +----- day of week (0 - 6) (Sunday=0)
| | | +------- month (1 - 12)
| | +--------- day of month (1 - 31)
| +----------- hour (0 - 23)
+------------- min (0 - 59)

.

0 0 * * *

This would happen exactly midnight on every day.

* * 1 * *

This would happen every hour, of every day, of January