What is a Julian date and a modified Julian date?
It’s the number of days since noon 4713 BC January 1. What’s so special about this date? Joseph Justus Scaliger (1540–1609) was a noted Italian-French philologist and historian who was interested in chronology and reconciling the dates in historical documents. As many calendars were in use around the world this created the problem of which one to use. To solve this Scaliger invented his own era and reckoned dates by counting days. He started with 4713 BC January 1 because that was when solar cycle of 28 years (when the days of the week and the days of the month in the Julian calendar coincide again), the Metonic cycle of 19 years (because 19 solar years are roughly equal to 235 lunar months) and the Roman indiction of 15 years (decreed by the Emperor Constantine) all coincide. There was no recorded history as old as 4713 BC known in Scaliger’s day, so it had the advantage of avoiding negative dates. Joseph Justus’s father was Julius Caesar Scaliger, which might be why he called it the
