100 Prisoners and a clock seems far too easy on the prisoners, so the exact same question, except that they must change the clock by exactly 4 hours:
There are 100 prisoners who are given a chance at freedom. The prisoners are randomly picked to visit a room where there is only a nonfunctional wall clock with a knob for manually changing the time.
The rules are as follows:
The prisoners are to enter the room and move the clock exactly four (4) hours backwards or forwards. They must choose one and may not try to communicate with the others in any fashion (aside from changing the time).
On any visit, a prisoner can announce that all 100 prisoners have visited, but must be absolutely sure (he will be required to divulge his strategy to win everyone's freedom).
For each visit, the prisoner will be picked by spinning a 100-slot roulette wheel. Thus, the order will be completely random (Prisoner 5 might be chosen 100 times before Prisoner 99).
Additionally, the visits will also occur randomly (perhaps 100 in a day, or perhaps a week without visits) and the prisoners have no knowledge of any visits aside from their own.
The initial setting of the clock will also be unknown to the prisoners.
As always, they may discuss a strategy beforehand.
What is a strategy that eventually leads to the prisoners being freed? (with chance arbitrary close to 100%)
bonus: How many visits will it take on average?