The Gregorian calendar more accurately tracks Earth's revolution around the Sun, more so than the older Julian calendar, in which every fourth year (including end-of-century years) is a leap year. Because of this, the Julian calendar began to gradually drift with respect to the four seasons. Each season has been occurring earlier and earlier over time. Therefore when the Gregorian calendar was created in 1582, it eliminated this problem by specifying that end-of-century years are only leap years if divisible by 400. 