Out of interest, is there any reason *this* is an invalid answer:
Pop (3 hours): 24:00 -> 3:00
Al (7 hours): 3:00 -> 10:00
Bernie (8 hours): 10:00 -> 18:00 (explosion at 17:00)
Iggy (6 hours): 18:00 -> 24:00
The only thing that strikes me as a possibility is the fact that Iggy talks about being relieved, but there's nothing to suggest he was relieved *by one of the other guards*.