I'm really surprised that a third of the DMs that have of yet posted do not keep track of time in their campaign world. The world does not revolve around the adventurers, however much they may wish and strive to make it so.
Between campaign I often, almost regularly, roll (or handwave) problems developing, being dealt with, etc for most major locations. Entire adventure hooks may arrive - and be dealt with - prior to the adventurers moving into an area. I like to keep things consistent, and I like a nice, multi-year historical background prepared (even if only outlined) in case they happen to enter an area unexpectedly. This way there is always something going on somewhere. Even if they enter an area and little is occuring to interest them, rumors may have arrived with the caravan of troubles elsewhere in need of aid, and if they do not leave soon enough to deal with them, the troubles may either go away or become more than they can currently deal with. And often there are several possible areas of interest to them.
Keeping track of all this requires a calander system, requires keeping track of what the current day is for the adventurers. Of course, I actually tend to prefer world building and maintaining as much (and sometimes more) than GMing, so perhaps my view is a bit different from most.