Based on 365 days per year, it will increase by nearly one day every four years, and it will decrease by nearly one day when it is pushed to 100. Therefore, it is stipulated that this day will not be added to the whole year of AD, but to the whole year of 400. This leads to different years, including normal years and leap years.

Therefore, it is stipulated to add one day every four years in February to make up for the underestimated time in the past. The year to make up for the time difference is leap year.

The time of a year is long or short, and the specific length depends on the time for the earth to go around the sun. The scientific level in ancient China was very advanced. As early as 2000 years ago, it was measured that the time for the earth to go around the sun was more than 365 days. Later, after precise calculation by scientists, the exact time was 365 days, 5 hours, 48 minutes and 46 seconds.

Extended data

Average year: There are 365 days in a year, and a year with no leap day in the solar calendar (that is, February 29th) or no leap month in the lunar calendar (including leap days in leap years in the Gregorian calendar) is called an average year. The average solar year is 365 days, the average lunar year is 354 days or 355 days, and very few days are 353 days.

Leap year: A leap year is established to make up for the time difference between the number of days in a year and the actual period of revolution of the earth caused by man-made calendar regulations, and the year to make up for the time difference is a leap year. The number of years in A.D. can be divisible by 4 as a leap year, but the number of years with integer hundred (both digits are 0) must be divisible by 400 as a leap year.

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