This volume contains descriptions of a selection of the calendars of the world and their history. There are introductory chapters on the nature of calendars, their astronomical background and the history of writing and counting. There are also chapters on the week and on the history of calendar reform. There is a section on methods of converting dates in one calendar to dates in another which are suitable for implementation on a hand calculator or computer; these include a discussion of the calculation of the date of Easter. The book is intended for the general reader rather than for the specialist but contains much material not otherwise easily accessible.