I've been having a similar, but different problem. After finishing my initial 4hr monster fights & trick-or-treaters, it said there were no more monsters and the time remaining was 2h 02m. So, I sent my trick-or-treaters on their tasks and decided to come back to the game a bit after 2 hours to set the monster fights. However, when I returned 2+ hours later, the timer was at 1h 56m remaining - it had barely moved. I decided to just let it go and come back in 2 hours time to reset my trick-or-treaters again and (hopefully) the monster fights.... I just checked in and the counter is 1hr 52m minutes. I think it stops ticking down once I close my town. This is VERY frustrating.
Edit: I just went back to my town to check something else and the timer is now saying 3h 35m remaining. So it's jumping around on me too now.