Gregorian calendar
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The Gregorian calendar is the calendar that is used throughout the Western world. It is similar to the Julian calendar, but because the Julian Calendar added a leap year with an extra day every four years with no exceptions, the length of the Julian year was 365.25 days -- when the actual time it takes for the Sun to go around the Earth once is closer to 365.2425 days. This made the seasons get out of track, since the first day of spring was starting earlier and earlier before March 21 as the centuries went by -- until by the 1500s it was starting around March 11, when everyone knew March 21 was supposed to be the first day of spring. So what they did was move the calendar forward ten days, and at the same time to make sure it didn't happen again, they made an exception to the leap year for every year that ended in 00 -- unless it could be divided by 400. So the year 2000 was a leap year anyway, because it can be divided by 400.
It was first suggested by the Neapolitan doctor Aloysius Lilius, and was made official by Pope Gregory XIII, for whom it was named, on February 24, 1582.
People sometimes use the term N.S. or New Style to mean the Gregorian calendar, with Old Style (or O.S.) meaning the Julian calendar.
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