If the calendar doesn't come into play often enough then you're not getting the benefit of having a fictional calendar either and the effort of creating one is wasted. If you do make one, and don't really emphasise it, then you're players are confused when you occasionally bring it up. If you make one and do emphasize it, then you potentially have all kinds of weird questions to answer. If your years, weeks, months, days, etc. are substantially different than what we're used to, for instance, then you don't have any frame of reference about how long things are or how old people "really" are.
I'm going through some of these same issues with a homebrew I'm developing that is a shattered world, so doesn't have any rotation. I think the best way to do it is just assume a relatively earth-like calendar, but not really emphasize it in game.