The Julian calendar is important to historians because it was used worldwide for over 16 centuries, and in various parts of the world for another three centuries after that. Its important to genealogists because it was used to record events in many countries as recently as the early 1900s.

Converting from Julian-calendar dates to our current Gregorian-calendar dates appears to be straightforward. But a deeper look shows the subtle issues involved, such as double-dating, undetermined year starts, and birthdates that change over time.

This talk presents the Julian calendar by first giving a historic perspective of the Roman calendars from which it was derived. It then explains the workings of the Julian calendar, and the reforms that were made to convert it to the more accurate Gregorian calendar. It describes the implications of these reforms, and problems that they can cause for genealogists and historians.