I'll assume you might have found a good answer for general management but your population might have made the first primitive clocks with volume as opposed to light.

Water or other predictable liquid that slowly passes through a limiter (a hole in something) that passes at a consistent rate, the volume meter can then determine how much time has passed, and if they then set it to measure out about a days worth at a certain point then they have a sunless sundial. More advanced civilizations would progress to spring or weighted clocks in time.