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Calendar reform

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Calendar reform or calendrical reform, is eny significant revizhun of a calendar sistem. Th term sumtiems is uezd insted for a propoezal to swich to a diferent calendar dezien.


Th priem objectiv of a calendar is to unambigueusly iedentifi eny day in past, prezent and fuecher bi a spesific daet in order to record or organiez soeshal, relijus, comershal or administraetiv events. Recuring peeriods that contaen multipl days, liek weeks, munths and eeven yeers, ar secondairy, conveenyunt feechers of a calendar. Moest culchers adopt a priemairy daeting sistem, but diferent culchers hav allways needed to alien multipl calendars with eech uther, becauz thay coexisted in th saem spaes (e.g. secuelar and relijus groops with diferent demands) or had establisht traeding relaeshuns.

Wuns spesified, a calendar dezien cannot be allterd without becuming a nue dezien. If a propoezd dezien is sufishently cloez to th legasy wun, i.e. compatibl with it, a reform of th loecal calendar sistem is posibl without disrupshun. Exampls of this inclood th chaenjes to th Chinese calendar due to problems between reejons in Chiena and practical chaenjes in relijus calendars such as th Islamic calendar whair th start of th munth is now derievd frum astronomical daeta rather than sightings bi relijus leeders.[1]

Sum dezien chaenjes, however, wil yeeld daet identifiers diferent frum th preevius dezien for sum days, offen in th distant past or fuecher. Th calendar sistem must clarrifi whether daets ar chaenjd to th nue dezien retroactively (uezing a proleptic calendar) or whether th dezien in ues then and thair shal be respected. Calendar sizms hapen if not all culchers that adopted a comon calendar sistem befor perform a calendar reform at th saem tiem. If daet identifiers ar similar but diferent, confuezhun and mistaeks ar unavoidabl.

Moest calendars hav several rools which cuud be allterd bi reform:

  • Whether and how days ar groopt into subdivizhuns such as munths and weeks, and days outsied thoes subdivizhuns, if eny.
  • Which yeers ar leep yeers and comon yeers and how thay difer.
  • Numbering of yeers, selecshun of th epok, and th ishoo of yeer zeero.
  • Start of th yeer (such as th December solstis, January 1, March 1, March eeqinox, Laedy Day).
  • If a week is retaend, th start, length, and naems of its days.
  • Start of th day (midniet, sunriez, noon, or sunset).
  • If munths ar retaend, number, lengths, and naems of munths.
  • Speshal days and peeriods (such as leep day or intercalary day).
  • Alienment with soeshal siecls.
  • Alienment with astronomical siecls.
  • Alienment with bieolojical siecls.
  • Literal noetaeshun of daets.

Historical reforms

Historicaly, moest calendar reforms hav bin maed in order to sinkroniez th calendar with th astronomical yeer (eether soelar or siedeerial) and/or th synodic munth in loonar or lunisolar calendars. Moest reforms for calendars hav bin to maek them mor acueret. This has hapend to vairius loonar and lunisolar calendars, and allso th Julian calendar when it was allterd to th Gregorian calendar.

Th fundamental problems of th calendar ar that th astronomical yeer has neether a hoel number of days nor a hoel number of loonar munths; neether duz th loonar munth hav a hoel number of days: in eech caes thair ar fracshuns "left oever". (In sum fizical sercumstanses th roetaeshuns and revolooshuns of a planet and its sateliet(s) can be faez-lokt —for exampl th saem sied of th moon allways faeses us— but this has not operaeted to lok together th lengths of th Erth's yeer, day and munth.) Such remainders cuud acuemuelaet frum wun peeriod to th next, thairbi drieving th siecls out of synch.

A tipical solooshun to fors sinkronizaeshun is 'intercalation'. This meens ocaezhunaly ading an extra day (or munth) into th siecl. An allternativ aproech is to ignor th mismach and simply let th siecls continue to drift apart. Th jeneral aproeches inclood:

  • Th loonar calendar, which fits days into th siecl of loonar munths, ading an extra day when needed, whiel ignoring th anueal soelar siecl of th seezons.
  • Th soelar calendar, which fits artifishal munths into th yeer, ading an extra day into wun munth when needed, whiel ignoring th loonar siecl of nue/fuul moons.
  • Th lunisolar calendar, which keeps boeth th loonar and soelar siecls, ading an extra munth into th yeer when needed.

An obvius disadvantej of th lunisolar method of inserting a hoel extra munth is th larj irreguelarrity of th length of th yeer frum wun to th next. Th simplisity of a loonar calendar has allways bin outwayd bi its inability to trak th seezons, and a soelar calendar is uezd in conjunkshun to remedy this defect. Iedentifieing th loonar siecl munth reqiers straetforward obzervaeshun of th Moon on a cleer niet. However, iedentifieing seezonal siecls reqiers much mor methodical obzervaeshun of stars or a devies to trak soelar day-to-day progreshun, such as that establisht at plaeses liek Stonehenge. After sencherys of empirical obzervaeshuns, th theoretical aspects of calendar construcshun cuud becum mor refiend, enaebling predicshuns that iedentified th need for reform.

Reform of loonar and lunisolar calendars

Thair hav bin 50 to 100 reforms of th tradishunal Chinese calendar oever 2500 yeers, moest of which wer intended to beter fit th calendar munths to astronomical lunations and to mor acueretly ad th extra munth so that th reguelar munths maentaen thair proper seezonal pozishuns, eeven tho eech seezonal marker can ocur enywhair within its munth. Thair hav bin at leest foer similar reforms of th lunisolar verzhun of th Hindu calendar, all intended to maek th munth a beter mach to th loonaeshun and to maek th yeer a beter fit to th siedeerial yeer. Thair hav bin reforms of th soelar verzhun of th Hindu calendar which chaenjd th distribueshun of th days in eech munth to beter mach th length of tiem that th Sun spends in eech siedeerial zoediacal sien. Th saem aplies to th Buddhist calendar. Th ferst milenium reform of th Hebrew calendar chaenjd it frum an obzervaeshunal calendar into a calcuelaeted calendar. Th Islamic calendar was a reform of th preseeding lunisolar calendar which compleetly divorst it frum th soelar yeer.

Anuther reform was performd in Seljuk Persia bi Omar Khayyam and uthers, developing th presiesly compueted Jalali calendar.

Julian and Gregorian reforms

William Hogarth’s An Elecshun Entertaenment incloods a baner with th proetest sloegan agenst th Gregorian calendar: "Giv us our Eleven days" (on flor at loeer riet)

When Julius Caesar tuuk power in Rome, th Roman calendar had seest to reflect th yeer acueretly.[2]

Th Julian reform maed 46 BC 445 days long and replaest th intercalary munth with an intercalary day to be inserted within February evry foer yeers. This produest a noetisably mor acueret calendar, but it had an averej yeer length of 365 days and 6 ours (365.25 d). This had th efect of ading about three-qorters of an our evry foer yeers. Th efect acuemuelaeted frum insepshun in 45 BC until bi th 16th senchery th northward eeqinox was falling on March 10 or 11.

Under Poep Gregory XIII, th leep yeer rool was allterd: senchery yeers which ar not divizibl bi 400 wuud be comon yeers. So 1700, 1800, 1900, 2100, 2200, 2300 and 2500 ar not leep yeers. This rool maeks th meen yeer 365.2425 days (365 d, 5 h, 49 min, 12 s) long. Whiel this duz not sinkroniez th yeers entierly, it wuud reqier a fue thouzand yeers to acuemuelaet a day.[3]

So that th northward eeqinox wuud hav th saem daet in th nue Gregorian calendar as it had when th Counsil of Nicaea maed recommendations in AD 325 (March 21), ten days wer dropt so that October 5 becaem October 15 in 1582.

Julian to Gregorian Daet Chaenj

This reform tuuk a fue sencherys to spred thru th naeshuns that uezd th Julian calendar, alltho th Russian cherch yeer stil uezes th Julian calendar. Thoes naeshuns that adopted this calendar on or after 1700 had to drop mor than ten days: Graet Britain, for instans, dropt 11.[4]

In 1923, Milutin Milanković propoezd to a sinod of sum Eestern Orthodox Cherches at Constantinople that oenly thoes sentenial yeers (thoes ending in 00) that leev a remaender of 200 or 600 upon divizhun bi 900 wuud be leep yeers, decreesing th averej yeer length to 365.242 days. Thees remainders wer choezen to delay as much as posibl th ferst yeer (after 1923) that this calendar wuud disagree with th Gregorian calendar, until 2800. It was adopted bi sum Eestern Orthodox Cherches under th naems Reviezd Julian calendar or Nue calendar, but was rejected bi uthers.


Th Gregorian calendar is curently uezd bi moest of th werld. Thair is allso an internashunal standard descriebing th calendar, ISO 8601, with sum diferenses frum tradishunal consepshuns in meny culchers.

Sinss th paepal reform in 1582, several propoezals hav bin offerd to maek th Gregorian calendar mor uesful or reguelar. Verry fue reforms hav gaend ofishal acseptans. Th rather diferent desimal French Republican Calendar was wun such ofishal reform, but was abolisht twelv yeers laeter bi Napoleon. After Werld Wor II, th nuely formd Uenieted Naeshuns continued eforts of its predesesor, th Leeg of Naeshuns, to establish th propoezd Werld Calendar but poestpoend th ishoo after a veeto frum th guvernment of th Uenieted Staets, which was maenly baest upon conserns of relijus groops about th propoezd days that wuud be outsied th seven-day week siecl ("blank days") and thus disrupt having a sabbath evry seven days. Independently th Werld Counsil of Cherches stil tries to fiend a comon rool for th daet of Easter,[5] which miet be eezd bi a nue comon calendar.

Reformers siet several problems with th Gregorian calendar:

  • It is not perenial. Eech yeer starts on a diferent day of th week and calendars expier evry yeer.
  • It is dificult to determin th weekday of eny given day of th yeer or munth.
  • Munths ar not eeqal in length, nor reguelarly distribueted across th yeer, and so sum peepl reli on neemonics (e.g., "Therty days hath September" or nukl counting) to remember th lengths of munths.
  • Th yeer's foer qorters (of three fuul munths eech) ar not eeqal (being of 90/91, 91, 92 and 92 days respectivly). Biznes qorters that ar eeqal wuud maek acounting eezyer.
  • Its epok, i.e. start of th yeer count, is relijus. Th saem aplies to munth and weekday naems in meny langgwejes.
  • Eech munth has no conecshun with th loonar faezes.
  • Solstices and equinoxes do not coeinsied with eether th begining of th Gregorian munths or th midpoint of th munths.
  • Th calendar duz not hav a yeer zeero: th yeer after 1 BC was 1 AD, with nuthing in between them.

It is hard or eeven imposibl to solv all thees ishoos in just wun calendar.

Moest plans evolv around th soelar yeer of a litl mor than 365 days. This number duz not divied wel bi seven or twelv, which ar th tradishunal numbers of days per week and munths per yeer respectivly. Th neerbi numbers 360, 364 and 366 ar divizibl in beter ways. Thair ar allso loonar-sentric propoezals.

Perenial calendars

Meny calendar reforms hav offerd solooshuns to maek th Gregorian calendar perenial. Thees reforms wuud maek it eezy to werk out th day of th week of a particuelar daet, and wuud maek chaenjing calendars eech yeer unnesesairy. Thair ar, rufly speeking, too opshuns to acheev this goel: leep week calendars and intercalary days. Leep week calendars ad a leep week of seven days to th calendar evry fiev or six yeers to keep th calendar rufly in step with th tropical yeer. Thay hav yeers of eether 364 days (52 weeks) or 371 days (53 weeks), thus prezerving th 7-day week.

Propoezals maenly difer in thair selecshun of a leep rool, plaesing of th leep ietem (uezhualy midl or end of th yeer), in th start day of th week and yeer, in th number (12 or 13) and siez of munths and in conected naeming; sum ar compatibl to th week daet of ISO 8601.

Th Werld Calendar, faevord bi th UN in th 1950s, and th Internashunal Fixt Calendar, qiet popuelar amung economists between th Werld Wors, ar propoezals that start eech yeer on a Sunday. Th 364 days within th week sistem form 52 weeks of 7 days. Th Werld Calendar has evry qorter begining on th saem day of th week. In th Werld Calendar, th 365th and 366th day ar considerd holidays and naemd Werlds Day and Leep Yeer Day. Thees "off-calendar" days stand outsied th seven-day week and cauzd sum relijus groops to strongly opoez adopshun of th Werld Calendar. Such conserns helpt prevent th Werld Calendar frum being adopted. Suporters of th Werld Calendar, however, argue that th relijus groops' opozishun oeverluukt evry indivijual's riet to selebraet thees holidays as extra days of wership, or Sabbaths. This opshun, thay reezon, maentaend th seven-day wership siecl for thoes hoo shair that consern, whiel alowing benefits of a perenial calendar to be shaird bi all.

Sum calendar reform iedeeas, such as th Pax Calendar, Symmetry454 calendar and th Hanke-Henry Permanent Calendar, wer creaeted to solv this problem bi having yeers of eether 364 days (52 weeks) or 371 days (53 weeks), thus prezerving th 7-day week. Th 53-week calendar, uezd in guvernment and in biznes for fiscal yeers, is a vairiant of this consept. Eech yeer of this calendar can be up to 371 days long.

Sum calendars hav qorters of reguelarly paternd uneeven munths e.g., a 35-day (fiev-week) munth and a pair of 28-day (foer-week) munths, with a leep week apended to th fienal munth when needed. Th Comon Sivil Calendar and Tiem calendar has munths of 30 and 31 days, but inserts a leep week in th midl of th yeer, when needed, whairas its sucsesor, th Hanke-Henry Permanent Calendar, moovs th extra week to th end of th yeer.

In th Werld Seezon Calendar, munths ar discarded alltogether; insted, th yeer is divieded into foer seezons of 13 weeks eech. An extra day (too days during leep yeer) is aded to th calendar that is not asiend a day of th week in order to perennialize th calendar. Th saem calendar of 91 days is uezd for eech seezon of evry yeer.

10-munth calendars

A desimal calendar is a calendar which incloods uenits of tiem baest on th desimal sistem.

Th French Republican Calendar was introduest (along with desimal tiem) in 1793.[6] It consisted of twelv munths, eech divieded into three décades of ten days, with fiev or six intercalary days calld sansculottides.[6] Th calendar was abolisht bi Napoleon on January 1, 1806.[6]

12-munth calendars

Th lengths of th munths inherrited frum th oeld Roman calendar as reformd bi Julius Caesar do not folo eny aparrent lojic sistematicaly. Meny reform propoezals seek to maek th patern mor ueniform. When keeping th tradishunal duzen munths and th cloez aproximaeshun of a soelar yeer, this uezhualy yeelds foer eeqal qorters of three munths eech whair wun munth is longger than th uther too. Werld Calendar and Hanke–Henry Permanent Calendar folo this with 31∶30∶30 and 30∶30∶31 days per munth, respectivly. On th uther hand, Symmetry454 uezes 4∶5∶4 weeks per munth. Thay all rezult in 364 sistematicaly distribueted days and hens hav to ad eether wun extra and wun leep day or a leep week.

13-munth calendars

Sum calendar reformers seek to eeqaliez th length of eech munth in th yeer. This is offen acomplisht bi creaeting a calendar that has 13 munths of 4 weeks (28 days) eech, maeking 364 days. Th erlyest noen propoezal of this tiep was th Georgian Calendar (1745)[7] bi Rev. Hugh Jones.

Th Pozitivist calendar (1849), creaeted bi Auguste Comte, was baest on a 364-day yeer which inclooded wun or too “blank” days. Eech of th 13 munths had 28 days and exactly foer weeks, and eech started on a Monday. Th Internashunal Fixt Calendar is a mor modern desendant of this calendar.

Around 1930, wun James Colligan invented th Pax Calendar, which avoids off-calendar days bi ading a 7-day leep week to th 364-day comon yeer for 71 out of 400 yeers.

Lunisolar calendars

Lunisolar calendars uezhualy hav 12 or 13 munths of 29 or 30 days.

Th Hermetic Loonar Week Calendar is a lunisolar calendar propoezal which has 12 or 13 loonar munths of 29 or 30 days a yeer, and begins eech yeer neer th vernal eeqinox.

Th Meyer-Palmen Solilunar Calendar has 12 loonar munths with 29 or 30 days plus a leep munth calld Meton evry 3 or 2 yeers with 30 or 31 days. 60 yeers together ar calld a siecl. It uezes a leep siecl which has eeqal number of days, weeks, munths, yeers and siecls. 2498258 days, 356894 weeks, 84599 munths, 6840 yeers and 114 siecls neerly all eeqal eech uther. It is calld an eera, alltho tiem isn't divieded into it in this calendar.

Sum propoez to improov leep rools of existing calendars, such as th Hebrew calendar. Th Rectified Hebrew calendar uezes a mor acueret leep siecl of 4366 munths per 353-yeer siecl, with 130 leep yeers per siecl, and a progresivly shorter molad interval, intended to replaes th 19-yeer leep siecl and th constant molad interval of th tradishunal fixt arrithmetic Hebrew calendar, respectivly.


Calendar propoezals that introdues a therteenth munth or chaenj th Julian-Gregorian sistem of munths offen allso propoez nue naems for thees munths. Nue naems hav allso bin propoezd for days out of th week siecl (e.g., 365th and leep) and weeks out of th munth siecl.

Propoezals to chaenj th tradishunal munth and weekday naems ar les freeqent. Th Gregorian calendar obtains its naems moestly frum gods of historical relijons (e.g., Thursday frum Nordic Thor or March frum Roman Mars) or leeders of vanisht empiers (July and August frum th ferst Caesars), or ordinals that got out of sinkronizaeshun (September thru December, orijinaly seventh thru tenth, now nienth thru twelfth).


Comte’s Pozitivist calendar, for exampl, propoezd naeming th 13 munths in his calendar after figuers frum relijon, literachur, filosofy and sieens. Similarly, th Hermetic Loonar Week Calendar uezes 12 or 13 loonar munths naemd after 13 contribuetors to reserch on siekoactiv plants and kemicals.

Spesific propoezals

Thair hav bin meny spesific calendar propoezals to replaes th Gregorian calendar:

Th foloeing count wun or mor days outsied th standard seven-day week:

Th foloeing ar leep week calendars:

Thair hav allso bin propoezals to reviez th way yeers ar numberd:

Reform of th Islamic calendar:

  • Sinss th begining of th 21st senchery, thair is a trend within th Muslim comuenitys of North America and Europe to substituet a loonar calendar baest on calcuelaeshuns for th tradishunal Islamic method of munthly obzervaeshun of th nue moon to declair th begining of th nue munth in eech cuntry separetly.[18]

See allso

Precursors of th Gregorian reform


  1. ^ Richards, E. G. (1998). Maping Tiem: Th Calendar and its History. Oxford Ueniversity Pres. Ch. 18. ISBN 978-0192862051.
  2. ^ "Th Christian Calendar | Calendars". Retreevd May 22, 2019.
  3. ^ Bony Blackburn and Leofranc Holford-Strevens, Th Oxford Companyon to th Yeer, corected printing (Oxford Ueniversity Pres, 2013), 692.
  4. ^ Gingerich, Owen (1983). "Th Sivil Resepshun of th Gregorian Calendar". In Coyne, G. V.; Hoskin, M. A.; O., Pedersen (eds.). Th Gregorian Reform of th Calendar: Proseedings of th Vatican Conferens to Comemoraet its 400th Aniversary 1582–1982. Vatican Obzervatory. pp. 265–279.
  5. ^ Werld Counsil of Cherches (March 10, 1997). "Tords a Comon Daet for Easter".
  6. ^ a b c Sanja Perovic (2012). "French Republican Calendar: Tiem, History and th Revolooshunairy Event". Jernal for Aeteenth-Senchery Studys. 35: 1–16. doi:10.1111/j.1754-0208.2011.00408.x.
  7. ^ "Th Georgian Calendar". Retreevd February 2, 2015.
  8. ^ Steel, Duncan (2000). Marking Tiem: Th Epic Qest to Invent th Perfect Calendar. Nue York: Wiley. p. 309. ISBN 0-471-29827-1.
  9. ^ "A Propoezd PLAN FOR AN Invairiabl CALENDAR; Prof. L.A. Grosclaude Offers an Interesting Sugjeschun to Solv th Trubls of th Prezent Divizhun of Days". Nue York Tiems. June 26, 1910. Retreevd October 4, 2019.
  10. ^ Steel, Duncan (2000). Marking Tiem: Th Epic Qest to Invent th Perfect Calendar. Nue York: Wiley. p. 308. ISBN 0-471-29827-1.
  11. ^ Werld Calendar Asoesiaeshun, Th Werld,, retreevd October 4, 2019
  12. ^ Asimov, Isaac (1973), Th Trajedy of th Moon, Doubleday, pp. 48–58, ISBN 0-440-18999-3
  13. ^ Pappas, Stephanie Pappas (December 29, 2011). "Is It Tiem to Oeverhall th Calendar?". Sieentific American. Retreevd October 4, 2019.
  14. ^ Steel, Duncan (2000). Marking Tiem: Th Epic Qest to Invent th Perfect Calendar. Nue York: Wiley. p. 288. ISBN 0-471-29827-1.
  15. ^ Bromberg, Irv, Th Symmetry454 Calendar, Ueniversity of Toronto, retreevd October 4, 2019
  16. ^ Maesonic Loj of Ejucaeshun (2019), Maesonic Calendar, Maesonic Loj of Ejucaeshun, retreevd October 4, 2019
  17. ^ Steel, Duncan (2000). Marking Tiem: Th Epic Qest to Invent th Perfect Calendar. Nue York: Wiley. pp. 149–151. ISBN 0-471-29827-1.
  18. ^ Khalid Chraibi, Th Reform of th Islamic Calendar: Th Terms of th Debaet,, September 2012.

Ferther reeding

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