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. In an amazing leap, the ancient Maya managed to synchronize all the visible planets too. In this tiny recycled guide to ancient calendars, Geoff Stray explains the difference between lunar and solar calendars, the importance precession, and why the Maya thought the current era would end in December 2012.