What is this?
Julius Caesar (with aid of Sosigenes of Alexandria) introduced this calendar to replace the earlier Roman calendar, which required manual addition of leap months to keep it in synchrony with the seasons. Unfortunately, it had leap days every four years without question, resulting in the average year having 365.25 days. However, the actual tropical year is roughly 365.2422 days, resulting in the Julian calendar gaining a day every 129 years. This means that seasons drift, starting earlier and earlier in the year.
In 1582, Pope Gregory XIII decided to fix this drift by reducing the number of leap days, motivated by a desire to keep the March equinox on March 21st, since that value was hardcoded in the calculation for the date of Easter. To achieve this, he made years divisible by 100 but not by 400 non-leap years, resulting in 97 leap years every 400 years, resulting in the Gregorian calendar. To bring the equinox back in alignment, October 5th to October 14th in 1582 were deleted. The result is the Gregorian calendar.
This website extends the Julian calendar indefinitely into the future for reference reasons. Note that in the 20th and 21st centuries, the Julian calendar is 13 days behind the Gregorian calendar.
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.