Ever since mankind first began to count, people have clocked days, moons and years, tried to make sense of them in terms of each other, and formed calendars to order their lives. Then, not so very long ago, the Maya made an amazing leap, and included the visible planets too. In this tiny recycled guide to ancient calendars, Geoff Stray explains the difference between lunar and solar calendars, the importance of precession, and why the Maya thought the current era would end in December 2012.