Calendar world for civil purposes. It is a purely solar calendar, with a cycle of leap days in a 400-year cycle designed to keep the duration of the year aligned with the solar year. Each Gregorian year has either 365 or 366 days (the leap day being inserted as 29 February), amounting to an average Gregorian year of 365.2425 days (compared to a solar year of 365.2422 days). It was introduced in 1582 as a refinement to the Julian calendar which had been in use throughout the European Middle Ages, amounting to a 0.002% correction in the length of the year.
February 29 − days or 365.2425 days. The length of the tropical year in 2000 was 365.24217 mean solar days, Adding a calendar day every four years, therefore, results in an excess of around 44 minutes every four years, or about 3 days every 400 years. To compensate for this, three days are removed every 400 years. The Gregorian calendar reform implements this adjustment by making an exception to the general rule that there is a leap year every four years. Instead, a year divisible by 100 is not a leap year unless that year is also divisible by 400. This means