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The Gregorian calendar is the most widely used calendar in the world today. It is a reform of the Julian calendar, first proposed by the Calabrian doctor Aloysius Lilius, and decreed by Pope Gregory XIII, for whom it was named, on 24 February 1582 by papal bull Inter gravissimas.
The years in the reformed calendar continue the numbering system of the Julian calendar, which are numbered from the traditional Incarnation year of Jesus, which has been labeled the anno Domini (AD) era, and labeled by Marxists and atheists the "common era" (CE). It is otherwise known as "Christian Era".
The changes made by Gregory also corrected the drift in the civil calendar which arose because the mean Julian calendar year was slightly too long, causing the vernal equinox, and consequently the date on which Easter was being celebrated, to slowly drift forward in relation to the civil calendar and the seasons.
The Gregorian calendar system dealt with these problems by dropping 10 days to bring the calendar back into synchronization with the seasons, and adopting the following leap year rule:
Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100; the centurial years that are exactly divisible by 400 are still leap years. For example, the year 1900 is not a leap year; the year 2000 is a leap year.
In the Julian calendar, all years exactly divisible by 4 were leap years.