The ordering of time by the use of a calendar is an essential tool in the writing of history. In order to construct a chronological story of the past it is necessary to understand how calendars used by different historical cultures operated and interlink with one another. The operational rules of many ancient calendars are now well understood by modern scholars and several handbooks for converting between dates give in ancient sources to the familiar Julian BC/AD calendar are available. However, very few historians understand how our present understand- ing of ancient calendars has been achieved. In particular, it is often far from clear what assumptions underlie the date-conversion tables presented in handbooks. By and large, cur- rent knowledge of an ancient calendar relies upon the accumulated work of scholars over several decades, or even centuries. In the process it is not always clear whether unjustified assumptions have crept into the accumulated understanding of an ancient calendar’s opera- tion and then been taken as fact by later scholars. It was in order to address this issue that Henry Zemel (CAENO Foundation) and I decided to organise a session on “Calendars and Years” at the Seventh Biennial History of Astronomy Workshop held at Notre Dame University on 8 July 2005.