Gregorian calendar

Internationally accepted civil calendar / From Wikipedia, the free encyclopedia

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The Gregorian calendar is the calendar used in most parts of the world.[1][lower-alpha 1] It went into effect in October 1582 following the papal bull Inter gravissimas issued by Pope Gregory XIII, which introduced it as a modification of, and replacement for, the Julian calendar. The principal change was to space leap years differently so as to make the average calendar year 365.2425 days long, more closely approximating the 365.2422-day 'tropical' or 'solar' year that is determined by the Earth's revolution around the Sun.

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2024 in various calendars
Gregorian calendar2024
MMXXIV
Ab urbe condita2777
Armenian calendar1473
ԹՎ ՌՆՀԳ
Assyrian calendar6774
Baháʼí calendar180–181
Balinese saka calendar1945–1946
Bengali calendar1431
Berber calendar2974
British Regnal year2 Cha. 3  3 Cha. 3
Buddhist calendar2568
Burmese calendar1386
Byzantine calendar7532–7533
Chinese calendar癸卯年 (Water Rabbit)
4721 or 4514
     to 
甲辰年 (Wood Dragon)
4722 or 4515
Coptic calendar1740–1741
Discordian calendar3190
Ethiopian calendar2016–2017
Hebrew calendar5784–5785
Hindu calendars
 - Vikram Samvat2080–2081
 - Shaka Samvat1945–1946
 - Kali Yuga5124–5125
Holocene calendar12024
Igbo calendar1024–1025
Iranian calendar1402–1403
Islamic calendar1445–1446
Japanese calendarReiwa 6
(令和6年)
Javanese calendar1957–1958
Juche calendar113
Julian calendarGregorian minus 13 days
Korean calendar4357
Minguo calendarROC 113
民國113年
Nanakshahi calendar556
Thai solar calendar2567
Tibetan calendar阴水兔年
(female Water-Rabbit)
2150 or 1769 or 997
     to 
阳木龙年
(male Wood-Dragon)
2151 or 1770 or 998
Unix time1704067200 – 1735689599
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The rule for leap years is:

Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400. For example, the years 1700, 1800, and 1900 are not leap years, but the year 2000 is.

There were two reasons to establish the Gregorian calendar. First, the Julian calendar assumed incorrectly that the average solar year is exactly 365.25 days long, an overestimate of a little under one day per century, and thus has a leap year every four years without exception. The Gregorian reform shortened the average (calendar) year by 0.0075 days to stop the drift of the calendar with respect to the equinoxes.[3] Second, in the years since the First Council of Nicaea in AD 325,[lower-alpha 2] the excess leap days introduced by the Julian algorithm had caused the calendar to drift such that the (Northern) spring equinox was occurring well before its nominal 21 March date. This date was important to the Christian churches because it is fundamental to the calculation of the date of Easter. To reinstate the association, the reform advanced the date by 10 days:[lower-alpha 3] Thursday 4 October 1582 was followed by Friday 15 October 1582.[3] In addition, the reform also altered the lunar cycle used by the Church to calculate the date for Easter, because astronomical new moons were occurring four days before the calculated dates. Whilst the reform introduced minor changes, the calendar continued to be fundamentally based on the same geocentric theory as its predecessor.[4]

The reform was adopted initially by the Catholic countries of Europe and their overseas possessions. Over the next three centuries, the Protestant and Eastern Orthodox countries also gradually moved to what they called the "Improved calendar", with Greece being the last European country to adopt the calendar (for civil use only) in 1923.[5] However, many Orthodox churches continue to use the Julian calendar for religious rites and the dating of major feasts. To unambiguously specify a date during the transition period (in contemporary documents or in history texts), both notations were given, tagged as 'Old Style' or 'New Style' as appropriate. During the 20th century, most non-Western countries also adopted the calendar, at least for civil purposes.

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