When the machine is turned on you can't go back before the time it was switched on. (Day 0) but you could go back to the exact moment it was switched on but not before.
The bit that worries me is when they switch it on, because you don't know what its capable of. Also the guy said people from the future could visit us - but they might be some nut jobs who have some super destructive weapons and kill us all therefore no future?
Lol, the machine would take ALOT and I mean ALOT of power to power it up... but the problem is, would they be able to turn it off?