Leap Year
leap year
A leap year is a calendar year containing an extra day (February 29), extending it to 366 days in order to keep the calendar aligned with Earth's orbital period around the Sun.
Gregorian Calendar
gregorian calendar
Overview
The Gregorian calendar is a solar calendar introduced by Pope Gregory XIII in 1582. It refined the Julian calendar's leap-year rule so that the average year length over 400 years is 365.2425 days, reducing the drift relative to the tropical year (365.2422 days) to just one day in 3,236 years. Today it is the internationally recognized civil calendar in virtually every country.
The Calendar Reform
The Julian calendar (established 46 BC) assumed a year of 365.25 days, but the small annual error of about 11 minutes and 14 seconds accumulated to roughly 10 days over 1,600 years. This drift caused problems for computing the date of Easter. Pope Gregory XIII corrected it by decreeing that October 4, 1582 would be immediately followed by October 15 (removing 10 days) and by modifying the leap-year rule for century years.
Adoption Around the World
Catholic countries adopted the reform in 1582, but Protestant and Orthodox nations resisted what they saw as a papal calendar. Britain switched in 1752 (dropping 11 days), Japan in 1873 (jumping from the traditional lunisolar calendar), Russia in 1918 (dropping 13 days), and Greece in 1923. When handling historical dates in software, developers must account for which calendar was in effect at a given time and place.
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