Otherwise, I lose track. What day of the week is it? How many weeks have passed since the opening scene? What season is it now? What holidays are coming up? Should the characters be wearing shorts or parkas by now?
To this end, I save the free calendars that come in my junk mail, and I use them to help with this aspect of plot and setting. Since I have never specified a particular year in which my books occur, I can pick any year. I'm not looking for my dates to match a specific year; I'm only looking for the relationship between scenes to make sense. A scene that happens seven days after a Sunday should also be on a Sunday; a scene that happens six months after midwinter should take place in midsummer, and so forth.