The time that you are setting is the length of time that the receiver is staying powered up internally, by setting it to 12 or 24 hours then it is still tracking the satellites the next day when you are ready to start and will have full accuracy much quicker if not immediately. 

On the 2630 you can put the display into Standby mode which doesn't shut it off but darkens it, then just touch the screen and it wakes up .