A calendar is simply a way to mark time, a system we have devised - and changed - over centuries to plan the future, remember the past and coordinate our lives. It is any system for dividing time over extended periods, and lets us know when religious holidays fall, when to celebrate a birthday or anniversary, when bills and taxes are due, when to go to work and how old we are. It gives us a temporal context in which to live and a way to mark the progress of humankind. It shapes us and - to a certain degree - control us.