If you think leap years simply happen every four years, think again!

*Note: This post was first published four years ago for Rossini's 54th birthday. I've changed the numbers to fit 2020 calculations.*

Today we celebrate Gioachino Rossini, born in Pesaro, Italy in 1792. By normal math, the famed opera composer and eccentric would be 228 years old today. But with Rossini it’s not so simple. Because he was born on February 29, i.e. leap day, we need to employ some special math to calculate his birthday. So today, instead of 228, we say that Rossini is 55.

But Chris! That math doesn’t add up! 228 years divided by 4 years doesn’t equal 55.

You’re right! It’s 57. Read on.

Leap years are not as simple as you may think. According to Eva Bodkin-Kowacki at the Christian Science Monitor this is how it works.

Contrary to what you’ve been told, one year is not 365 days long; it is 365.242159 days long. And so in order to accommodate this approximately quarter extra day each year, the Gregorian (and Julian) calendar adds one day every four years, February 29. That’s not all though. This is where the special math comes in.

Because one year is a little less than quarter day longer each year, every centennial year we skip leap day all together and have a normal year:

1792 = 366 days

1796 = 366 days**1800 = 365 days**

1804 = 366 days

Since Rossini’s birthday in 1792, there have been three centennials: 1800, 1900, and 2000.

But Chris! 228 years divided by 4 years and then minus 3 centennial years *still* doesn’t equal 55.

Correct again! Read on.

The centennial thing isn’t a perfect fix for the 365.242159 day year either. It’s close, but not perfect. One more accommodation needs to be made: every centennial year *that is divisible by 400 *does not skip the leap day, and February 29 is added back in:

1800 = 365 days

1900 = 365 days**2000 = 366 days**

2100 = 365 days

Since 2000 is divisible by 400, it is counted toward the total number of Rossini birthdays: 55.

LONG STORY SHORT

- 2020 minus 1792 = 228 years
- 228 divided by 4 = 57
- 57 minus 3 for the centennial years 1800, 1900, and 2000 = 54
- Add one year back for 2000, a centennial year divisible by 400 = 55

And voila! Rossini is 55 years young today. Now, if your brain is steaming as much as mine was, then enjoy this to cool it off: