<Tr> <Td> Tibetan calendar </Td> <Td> 阴 火 鸡 年 (female Fire - Rooster) 2144 or 1763 or 991--to--阳 土 狗 年 (male Earth - Dog) 2145 or 1764 or 992 </Td> </Tr> <Tr> <Td> Unix time </Td> <Td> 1514764800--1546300799 </Td> </Tr> <P> The Gregorian calendar is the most widely used civil calendar in the world . It is named after Pope Gregory XIII, who introduced it in October 1582 . The calendar spaces leap years to make the average year 365.2425 days long, approximating the 365.2422 day tropical year that is determined by the Earth's rotation around the Sun . The rule for leap years is as follows: </P> <P> Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400 . For example, the years 1700, 1800, and 1900 are not leap years, but the year 2000 is . </P>

When did we start using the modern calendar