The time controls what time The most common application is to control the time when the sprinklers come on as mandated by the certain areas. The timer allows for the the unit to spray at a specific time over and over with out any input. Since the time is basically a relay over time it wear down from the electrical loan it take and needs to be replaced. If your panel has a Grasslin timer in it this will directly replace it. This unit has a built in battery for backup should you lose power.