What is a leap year and why do we have it?

Every four years, there is a year with 366 days in it.
The extra day is added at the end of February, which is why February 29th is also known as leap day.

A leap year is a year in the Gregorian (Western) calendar that meets the following conditions:

1. Any year in which the western calendar year is divisible by 4 is a leap year.
2. However, a year in which the AD year is divisible by 100 is usually a normal year.
3. However, any year in which the Western calendar year is divisible by 400 is a leap year.

For example, the year 2000 is a leap year because it is divisible by 400. On the other hand, 1900 is a normal year because it is divisible by 100 but not by 400.

The main purpose of leap years is to correct the discrepancy between the solar calendar (a calendar based on the position of the sun) and the calendar we use in our daily lives (the Gregorian calendar).

The earth orbits the sun in approximately 365.24 days. Our calendar, however, has 365 days in a year. This results in a discrepancy of approximately 0.24 days each year. If this accumulates, after four years, a misalignment of approximately 0.24 days x 4 = 0.9688 days will occur between the solar and Gregorian calendar dates.

Leap years were invented to solve this problem. The addition of one day in a leap year compensates for this discrepancy. However, since an additional day is added every 4 years, that adds about 0.0312 days (1 day - 0.9688 days) more than the original misalignment.

To further compensate for this approximate 0.0312-day discrepancy, the exception rule for years divisible by 100 and divisible by 400, as described in "How to Calculate Leap Years" above, is applied.

This results in an average number of days in a year of 365.2425 in the Gregorian calendar, with leap years being set 97 times in every 400 years. This is a difference of about 0.0003 days from the length of a solar calendar year (about 365.2422 days), which is not exactly the same.

Without a leap year, our lives could be affected by the following problems:

1. Time displacement: Without a leap year, the discrepancy between the solar calendar year (approximately 365.2422 days) and the Gregorian calendar year (365 days) would accumulate and after a few years result in a significant shift regarding the start of the seasons.
2. Agricultural impact: Seasonal shifts could effect our agriculture significantly. A shift in the timing of crop planting and harvesting may affect the produce and quality of the crop.
3. Cultural and traditional influence: Many cultures and traditions are based on specific seasons or dates. Without a leap year, these dates would shift and the dates of traditional events and festivals would change inconsistently.
4. Impact on economic activity: Seasonal shifts may affect tourism, retail and other seasonally dependent industries.
5. Impact on daily life: In the long term, the seasonal shift may affect various aspects of our daily life, such as health precautions and clothing. The term "I have nothing to wear" would definitely be heard more often.

To identify the next leap year, apply the following rules and exceptions described in the "How to Calculate Leap Years" section above.

1. Any year in which the western calendar year is divisible by 4 is a leap year.
2. However, a year in which the AD year is divisible by 100 is usually a normal year.
3. However, any year in which the Western calendar year is divisible by 400 is a leap year.

Below you can find a list of all leap years in this century. 

The year 2100 is a normal year because it is divisible by 100 but not by 400.

People born on leap day, February 29, only get to have a real birthday once every four years, but they still get older every year. In Japan, those born on February 29 are usually celebrated on February 29 in leap years and on February 28 after 24:00 (March 1) in normal years as their birthday.