What is a Leap Year?
A leap year is one that contains 366 days versus a normal year that contains 365 days. This extra leap day is added to February, meaning the month of February has 29 days in a leap year whereas February in a non-leap year only has 28 days.
When is a Leap Year?
The extra day (February 29) is added to years where:
- The number 4 divides into the year evenly but the number 100 does not.
- Or, the number 400 divides into the year evenly.
How to Calculate Leap Year
To illustrate how to calculate a leap year using the above formula, we'll use three example years: 2000, 2019, and 2020.
2000: Since 4 does divide into 2000 evenly (2000 ÷ 4 = 500), but so does 100 (2000 ÷ 100 = 20), at first glance you might not think that 2000 is a leap year. However, since 400 does divide into evenly (2000 ÷ 400 = 5), 2000 is a leap year after all.
2019: Since 4 does not divide into 2019 evenly (2019 ÷ 4 = 504.75), 2019 is not a leap year.
2020: Since 4 does divide into 2020 evenly (2020 ÷ 4 = 505) and 100 does not divide into it evenly (2020 ÷ 100 = 20.2), 2020 is a leap year.
If you want to calculate leap year in Excel, here is one you can do it:
| A | B | |
|---|---|---|
| 1 | Year | Leap Year? (Formula) |
| 2 | 2000 | =OR( MOD(A2,400)=0, AND( MOD(A2,4)=0, MOD(A2,100)>0 ) ) |
Using the earlier example years, and filling the formula down Column B, here is what the spreadsheet would look like:
| A | B | |
|---|---|---|
| 1 | Year | Leap Year? |
| 2 | 2000 | TRUE |
| 3 | 2019 | FALSE |
| 4 | 2020 | TRUE |
Why Do We Need Leap Years?
The reason adding an extra day every four years (with some exceptions) is needed, is because it takes the earth roughly 365.24 days to travel around the sun (a solar year), whereas as the Gregorian calendar is based on a 365-day year.
Since a solar year is different from a Gregorian year, if we didn't add leap days to the calendar periodically, the calendar year would continually fall further and further behind the solar year -- causing our seasons to get out of whack.