Date of the first leap year not dating
The Chinese Calendar The calendar in official use today in most Western countries, and also in the Christian faith, is the Gregorian calendar. The Gregorian calendar is a solar calendar and tries run in step with the beginning of spring in the northern hemisphere (related to the Christian feast of Easter [Steel]) without keeping track of the phases of the Moon.That January 1st is now considered to be the first day of the year is a historical accident.If you thought the birth of Jezus or the midwinter feast (Yule) was the most important, then you picked 25 December.If you thought the circumcision of Jezus was the defining moment, or if you preferred to have the first day of the year at the beginning of a month, or if you wanted to keep to the tradition of the Romans, then you used 1 January.
A date in 1612, for example, is probably on the Gregorian calendar if it occurs on a letter sent from a Catholic country, but probably on the Julian calendar if it is on a letter sent from a Protestant country.
In the Gregorian calendar, 400 years are exactly equal to 146,097 days, and after such a period the sequence of ordinary and leap years repeats itself.
The average length of the calendar year is 365 97/400 = 365.2425 days, which is about 11 seconds longer than the average time from one spring to the next, and about 11 days longer than 12 synodical months.
The first year of the calendar is tied to the birth of Jesus (from the Christian faith).
The first day of the year, 1 January, falls just after the middle of the winter half od the year in the Northern hemisphere.