For example, we will store the date a player joined the city, and when it logged in last time. This way, we can do some nice things such as have referendums, where we can have decisions taken only by more than 50% of the number of people that logged in the last few days. This way, if half of the city players are in vacation, and the city needs a referendum, they can use the majority of the rest of the players (that is, a majority will be 25% of the total population).
On an unrelated note, I just slipped in the shower, and almost broke my left elbow and foot. It hurts, the bone is a little bumped, but at least not broken.