Documentation:Calendars

Let’s talk a bit about calendars used by the Timeline. As you probably know, the calendar reform that replaced Julian calendar with Gregorian calendar took place in 1582. If you look at October 1582 on the Timeline, you’ll see that October 4th is immediately followed by October 15th.

October 4, 1582 is the last Julian date, and October 15, 1582 is the first Gregorian date. As far as I know, any dates before the calendar reform that you can find in pretty much any historical literature are Julian dates. So, when it comes to the Timeline, it makes sense to use Julian calendar for dates up until the reform and Gregorian calendar for all the dates after the reform. That’s why when you enter dates earlier or equal to October 4, 1582, the program assumes that those are Julian dates. Dates from October 15, 1528 and onward are assumed to be Gregorian.

Put these instructions into the Editor:

Last day of Julian calendar;;;;10.04.1582;;
First day of Gregorian calendar;;;;10.15.1582;;

and you’ll get this:

If you open the Menu and select to show Julian dates after the reform, you’ll see that in Julian calendar those dates are just 4th and 5th of October.

If you use the sources with Old style dates (i.e. Julian dates after the calendar reform) you need to convert them into Gregorian dates. It is easy to do with the Parser.

A few facts about Julian calendar:

• People started using February 29th as a leap day only in the late Middle Ages. Before that they inserted an extra day after February 23rd. The Timeline doesn’t reflect that. We always use February 29th as a leap day on the Timeline.

• The leap years that were actually observed between the implementation of the Julian calendar in 45 BC and AD 8 were erratic, so we use a proleptic Julian calendar for anything before AD 8. Basically we just make every 4th year a leap year regardless of what happened in reality. In the Common era leap years are 4 AD, 8 AD, 12 AD and so on. Leap years before Common era are: 1 BC, 5 BC, 9 BC and so on.

• Year 46 BC (the year of Caesar’s calendar reform) had 445 days in it. A bunch of preceding years were much shorter than a usual year. In general Roman calendar before the reform had shorter month lengths and used and intercalary month instead of an intercalary day. The Timeline ignores all that. We just use normal 365 and 366 day years for all the years BC. This can be changed with future updates if such need ever arises.