# Happy February 29th! - Leap Year Facts

2024-02-29 - [52] 12:16

Today is the day that some time displays and date calculation programs break! To those who were born on February 29th, happy "true" birthday! This post from your resident "Calendar Nerd" is going to talk a little bit about Leap Years.

## Why we have leap years

The Earth revolves around the Sun in a bit longer than 365 days. The spin of the Earth around its own axis is not tied to the amount of time the Earth takes to revolve around the sun, but because we tend to keep track of time in days, it's useful to have a year be a round amount of days long. Rather than taking 365 days to revolve around the Sun, the Earth takes almost a quarter of a day longer to complete a full revolution. That means after about 4 years, calendars would be 1 day ahead of where they should be. The solution in our current calendar (the Gregorian Calendar) is to add a leap day every few years to keep the calendar mostly aligned to the tropical year, meaning the starts of seasons consistently stay within a 1 day window year to year.

## The "Leap Day" used to be on February 24th

When Julius Caesar created the Julian Calendar, which our current calendar system is heavily based on, he set up a leap day as "Ante diem bis sextum Kalendas Martii", which translates essentially to "Twice the 6th day before March". The end of the month was essentially a count-up towards March. Starting from February 23rd up to March 1st, we basically get a countdown of 6 (23rd), 5 (24th), 4 (25th), 3 (26th), 2 (27th), 1 (28th), but on a leap year, we get a second "6th day before March", so we get a countdown of 6 (23rd), 6' (24th), 5 (25th), 4 (26th), 3 (27th), 2 (28th), 1 (29th).

## Leap years aren't every 4 years

For most years, the rule of "if the year is divisible by 4, it's a leap year" holds true, but it's a bit more complicated than that. The system for the Julian Calendar, created and implemented by Julius Caesar before his assassination, had leap years occur every 4th year, but over about 1600 years, the calendar year drifted off of the tropical year by over 10 days. The calendar system we use now, the Gregorian Calendar, was implemented to fix this misalignment (although it tried to correct to about 325 AD/CE rather than 45 BC/BCE when the Julian calendar was implemented).

The Gregorian Calendar leap year rules are (for AD/CE dates):

- Years divisible by 400 ARE leap years
- All other years divisible by 100 ARE NOT leap years
- All other years divisible by 4 ARE leap years

That means 2024 AD/CE is a leap year, as was 2000 AD/CE, but 1900 AD/CE was NOT a leap year.

The Gregorian Calendar leap year rules for BC/BCE dates are:

- Convert year by subtracting the BC/BCE year from 1 (example: 3 BC/BCE becomes -2 because 1 - 3 = -2)
- Converted years divisible by 400 ARE leap years
- All other converted years divisible by 100 ARE NOT leap years
- All other converted years divisible by 4 ARE leap years

That means 5 BC/BCE was a leap year, as was 401 BC/BCE and 1 BC/BCE, but 101 BC/BCE was NOT a leap year.

## Leap years were every 3 years by accident for a few decades

After Julius Caesar was assassinated, it was up to the priests after him to make sure his calendar system worked properly. Before the Julian Calendar was implemented, it was up to the high priest to determine if a year needed about an extra month to realign the calendar year with the tropical year, but the Julian Calendar got rid of that system by making the length of a year very similar to a tropical year length and incorperating an automatic leap year rule. As mentioned in the previous section, the Julian Calendar's leap year system was to have every 4th year be a leap year. Unfortunately, leap years were applied every 3 years instead of every 4 years for about 36 years. Caesar Augustus called for leap years to not occur for about 12 years to get the Julian Calendar realigned to its correct values.

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