Explanation

What is this?

This is an implementation of the Gregorian calendar—which is the default civil calendar in most countries—with conversions to various other calendars.

Currently, it displays the following conversions for each day, if possible:

How are BCE years handled?

The astronomical convention is used, i.e. year 0 is 1 BCE, year -1 is 2 BCE, etc.

Due to the way the common era (a.k.a. Anno Domini) year numbering system works, the year 1 BCE is followed directly by 1 CE with no year zero. This makes math hard. As such, the astronomical convention of making 1 BCE year 0 and extending this into the past was used.

What is the JD (Julian day number) value?

The integer Julian day (JD) number is the continuous count of days since the beginning of the Julian period, which is very useful in astronomy and in software for calculating durations without worrying about weird calendar issues. In this respect, it's very similar to Unix time.

JD 0 is assigned to the date Monday, January 1, 4713 BCE (Julian) or November 24, 4714 BCE (Gregorian). More specifically, if fractional JDs are considered, then the integer value specifically refers to the Universal Time noon on that date.

What is the LC (Mesoamerican Long Count) date?

The five (or more) numbers separated by dots is the corresponding Mesoamerican Long Count calendar date. This is commonly known as the “Mayan calendar.” This calendar is not available for dates before August 11, 3114 BCE (Gregorian) or September 6, 3114 BCE (Julian).

What is the FR (French Republican calendar) date?

The French Republican calendar was a calendar created and implemented during the French Revolution. It is also frequently referred to as the French Revolutionary Calendar, but this is a misnomer: year 1 of the calendar started on 22 September 1792, the day after the abolition of the monarchy and the founding of the French First Republic.

Specifically, the variant used here is the one I computed from astronomy.