The Julian calendar is a calendar introduced by Julius Caesar in 46 BC and created by Caesar's resident expert in such matters, a Greek named Sosigenes, to replace the Roman calendar. Sosigenes set up the months as we now know them and added an extra day in February every fourth year (see leap year). This gives an average year of 365¼ days, which is pretty close to the period over which the seasons exactly repeat (the tropical year). However, there is an error of about three days every four centuries. By 1582, the calendar was about 10 days out of kilter with the real world. Pope Gregory XIII took two steps to deal with this. First, he decreed that October 4, 1582, would be followed immediately by October 15. Secondly, to keep the problem from recurring, he decreed that three out of every four century years (those ending in 00) would not be leap years. Thus, 1700, 1800, and 1900 were not leap years, even though they are divisible by 4, but 2000 was a leap year. The result was the Gregorian calendar used in most of the world today. Unfortunately, it took from 1582 to 1918 for the Julian calendar to completely die out, so astronomers need to make clear which system is being referred for dates in that interval. Civilian or astronomical dates based on the Julian calendar are said to be Old Style dates. Except for some religious purposes, the Julian calendar is now obsolete.