Leap year is not really every four years.
The way we measure time on Earth is by measuring the length of time it takes to orbit the sun, called “solar” year. The precise measure of solar year is 365.24219 days.
That might seem like an insignificant difference. By using the Julian calendar, which uses 365 ¼ days, this resulted to miscalculation of 11 minutes every year. By 1500s, Julian calendar and solar year were misaligned by about 10 days, so Gregorian calendar was established.
If year is NOT divisible by 4, then it is a common year.
Else if year is NOT divisible by 100, then it is a leap year.
Else if year is NOT divisible by 400, then it is a common year.
Else, it is a leap year.
Year 1900 was not a leap year, yet 2000 was a leap year. In 2100, it will be a common year.
- US Naval Observatory