JR Rail Pass validity

I will a 7 days Rail Pass but firstly I'd like to know:
is it valid from the exact hour of activation or it follows regular day?
I mean if for example I activate at 11 or 16 pm it counts as one day? So it will be expired at midnight of the seventh day and not at 11 or 16 pm of the seventh day?

Calender days to midnight, not 24 hour clock.

As the first reply says, it runs on calendar days, so will expire at midnight on the last day. If you're on a train at midnight, the pass is valid until you finish your journey (i.e. you can get off the train and leave the paid area of the station, but not re-enter).