How might an underground (dwarven, say) civilization isolated from the surface world organize their calendar/date system? I'm not talking about the means of keeping time, but how it is defined. Sick of seeing original settings like Menzoberranzan, against all logic, tracking the passage of time by days, months and years. Of course there is no real historical data point for this. But I only need a plausible suggestion, or even better, a few of them. How can one eliminate all measures derived from surface phenomena?