Definition of a leap year

What is a leap year? The first thing you think of is that the year that can be divided by four is a leap year. In fact, this is incorrect. The concept of a leap year in a Gregorian calendar is as follows: a year that can be divisible by 400, or cannot be divisible by 100, but can be divisible by 4, is a leap year. In other words, A leap year is not the year of the century that can be divided by four, and the year of the century that can be divided by 400. According to this definition, 2000 AD is a leap year, and 1900 AD is a normal year.
But if I ask you again, is 100 AD a leap year? This is the year of the century, and cannot be divided by 400. Therefore, this year is a year of the year. If you think so, you will be wrong.
The Gregorian calendar we use today is the Gregorian calendar, which was passed after 1582 AD. The previous recorded date uses the Gregorian calendar. Unlike today's Gregorian calendar, the definition of a leap year in the Confucian calendar is very easy, what can be divisible by four is a leap year, and what cannot be divisible by four is a normal year. Therefore, in 100 AD, the year was still in the Period of Confucianism, so the year in history was a leap year.
Therefore, the full definition of a leap year is: years that can be divided by four before 1582 AD, years that can be divided by 1582 AD after 400, and years that can be divided by four in the second century.
In general, the definition of a leap year is more complex than that of a leap year, far from being as simple as the average person thinks. This is not only a mathematical problem, but also a historical problem.