Joe Celko's SQL for Smarties, 3rd Edition

4.1. Notes on Calendar Standards

Leap years did not exist in the Roman or Egyptian solar calendars prior to the year 708 A.U.C. (ab urbe condita, Latin for “from the founding of the City [Rome]”).

Unfortunately, the solar year is not an even number of days; there are 365.2422 days in a year, and the fraction adds up over time. The civil and religious solar calendars had drifted with respect to the solar year by approximately one day every four years. For example, the Egyptian calendar drifted completely around approximately every 1,461 years. As a result, it was useless for agriculture, so the Egyptians relied on the stars to predict the flooding of the Nile. Sosigenes of Alexandria knew that the calendar had drifted completely around more than ...