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This article is a general overview of the calendar reform movement. For a list of specific proposals for calendar reform, see the article, Alternative and Proposed Calendars.

A calendar reform is any significant revision of a calendar system. The term is sometimes also used for mere proposals of new calendars and changeovers from one calendar system to another.

Most calendars consist of several rules, any of which may be altered by a reform:

  • The grouping of days of the year into months, weeks, or similar subdivisions, together with any days of the year not belonging to these subdivions.
  • A Leap year rule, determining which years are leap years, and how each leap year differs from the other years (common years)
  • The numbering of years, i.e. selection of the epoch and the issue of year zero.
  • The start of the year (e.g. winter solstice, January 1, March 1, spring equinox, Easter), week (usually Sunday or Monday, sometimes Saturday) and day (midnight, sunrise, noon, sunset).
  • The naming of months and weekdays, as well as other special dates and periods (e.g. leap day or intercalary day).
  • Alignment with religious and economic cycles as well as regular, recurring astronomic phenomena.
  • Literal notation of dates.

Historical reforms[]

Historically, most calendar reforms have been made in order to synchronize the calendar in use with the astronomical year (either solar or sidereal) and/or the synodic month in lunar or lunisolar calendars. Most reforms for calendars have been to make them more accurate. This has happened to various lunar and lunisolar calendars, and also the Julian calendar when it was modified into the Gregorian calendar.

Reform of lunar and lunisolar calendars[]

There have been 50 to 100 reforms of the traditional Chinese calendar over 2500 years, most of which were intended to better fit the calendar months to astronomical lunations and to more accurately add the extra month so that the regular months maintain their proper seasonal positions, even though each seasonal marker can occur anywhere within its month. There have been at least four similar reforms of the lunisolar version of the Hindu calendar, all intended to make the month a better match to the lunation and to make the year a better fit to the sidereal year. There have been reforms of the 'solar' version of the Hindu calendar which changed the distribution of the days in each month to better match the length of time that the Sun spends in each sidereal zodiacal sign. The same applies to the Buddhist calendar. The first millennium reform of the Hebrew calendar changed it from an observational calendar into a calculated calendar. The Islamic calendar was a reform of the preceding lunisolar calendar which utterly divorced it from the solar year.

Julian/Gregorian reforms[]

At the time when Julius Caesar took power in Rome, the Roman calendar had ceased to reflect the year accurately. The provision of adding an intercalary month to the year when needed had not been applied consistently, because it affected the length of terms of office.

The Julian reform lengthened the months (except February, owing to its religious significance) and provided for an intercalary day to be added every four years to February, creating a leap year.

This produced a noticeably more accurate calendar, but it was based on the calculation of a year as 365 days and 6 hours (365.25 d). In fact, the year is 11 minutes and 14 seconds less than that. This had the effect of adding three-quarters of an hour to a year, and the effect accumulated. By the sixteenth century, the vernal equinox fell on March 10.

Under Pope Gregory XIII the leap rule was altered: century years, which are divisible by four, would not be leap years unless they are also divisible by 400. This makes the mean year 365.2425 days (365 d, 5 h, 49 min, 12 s) long. While this does not synchronize the years entirely, it would require 35 centuries to accumulate a day. This new calendar was synchronized with the traditional seasons again and was not applied to dates in the past, which caused a leap of at least ten days from the final day the Julian calendar was in effect. This reform slowly spread through the nations that used the Julian calendar, although the Russian church year still uses the Julian calendar. The times varied so widely that some countries had to drop more than ten days: Great Britain, for instance, dropped eleven.

When noting dates occurring within the period, "Old Style" and "New Style" are used to distinguish which calendar was used by the person who recorded the date.


The Gregorian calendar is currently used by most of the world. There is also an international standard describing the calendar, ISO 8601, with some differences to traditional conceptions in many cultures.

Since the last papal reform, several proposals have been offered to make the Western calendar more useful or regular. Very few reforms, such as the rather different decimal French Republican and Soviet calendars, had gained official acceptance, but each was put out of use shortly after its introduction. After World War II the new-formed United Nations continued efforts of its predecessor, the League of Nations, to establish a new calendar but postponed the issue after a veto from the USA, which was mainly based upon concerns of religious groups about the proposed days out of the week cycle. Independently the World Council of Churches still tries to find a common rule for the date of Easter,[1] which might be eased by a new common calendar.

Reformers cite several problems with the Gregorian calendar:

  • It is not perpetual. Each year starts on a different day of the week and calendars expire every year.
  • It is difficult to determine the weekday of any given day of the year or month.
  • Months are not equal in length nor regularly distributed across the year, requiring mnemonics (e.g. “Thirty days hath September”) to remember which month is 28, 29, 30 or 31 days long.
  • The year’s four quarters (of three full months each) are not equal. Business quarters that are equal would make accounting easier.
  • Its epoch (origin) is not religiously neutral. The same applies to month and weekday names in many languages.
  • Each month has no connection with the lunar phases.
  • The intercalary day occurs after the second month instead of at the end of the year, which complicates calendrical calculations.
  • Intercalation occurs at 4-year or 8-year intervals. This causes astronomical events such as equinoxes to occur in a 53-hour range over a 400-year span.

It is impossible to solve all these issues in just one calendar.

Most plans evolve around the solar year of little more than 365 days. This number does not divide well by seven or twelve, which are the traditional numbers of days per week and months per year respectively. The nearby numbers 360, 364 and 366 are divisible in better ways. There are also lunar centric proposals.

Comparison of Proposed Solar Calendar Reforms (Gregorian/ISO Date Equivalents)
Author Wk New Year's Day Jan
13 Extra Calendrical Days Intercalary days
Pompilius 8 Variable day, (Feb 24) 29 23
31 29 31 29 31 29 29 31 29 29 (Feb 24) + Intercalaris 27 day month
Caesar 7 Variable day, Jan 1 31 28 31 30 31 30 31 31 30 31 30 31 Feb 29
Romme 10 W01.1,
N. Aut. Equinox, 01.01
30 30 30 30 30 30 30 30 30 30 30 30 Sansculottides (12.31-35) (12.36) (W37.8)
Comte 7 Monday, Jan 1 (01.01) 28 28 28 28 28 28 28 28 28 28 28 28 28 Leap D. (07.00), Year Day (13.29) Leap D. (07.00) (W27.0)
Emil Hanin 7 Monday, Jan 1 31 30 30 31 30 30 31 30 30 31 30 30 (12.31), (12.32) (12.32) (W52.8/W53.0)
Gaston Armelin 7 External (W00.0), (Jan 0/ 00.00) 30 30 31 30 30 31 30 30 31 30 30 31 New Years D. (00.00), Leap D. (12.32) Leap D. (12.32) (W52.8/W53.0)
L. A. Grosclaude 7 External (W00.0), (Jan 0/ 00.00) before Mon. 30 30 31 30 30 31 30 30 31 30 30 31 New Years D. (Jan 0/ 00.00), Leap D. (Jul 0/ 07.00) Leap D. (Jul 0/ 07.00) (W27.0)
Alexander Philip 7 External (W00.0), (Jan 0/ 00.00) before Sun. 30 30 31 30 30 31 30 30 31 30 30 31 I (Jan 0/ 00.00), II (Jul 0/ 07.00) II (Jul 0/ 07.00) (W27.0)
Colligan 7 Sunday, Jan 1 28 28 28 28 28 28 28 28 28 28 28 28 28 Leap Week 13.1-7 moving Dec from 13 to 14
Soviet 5 W01.1, 01.01 30 30 30 30 30 30 30 30 30 30 30 30 Lenin D. (01.31), Leap D. (02.31), Labor D. (04.31), Industry Days (11.08-10) Leap D. (02.31) (W12.6/W13.0)
Achelis 7 Sunday, Jan 1 31 30 30 31 30 30 31 30 30 31 30 30 World Leap D. (Jun 31/ 07.00), World D. (Dec 31/ 12.31) World Leap D. (Jun 31/ 07.00) (W27.0)
Asimov 7 Sunday, N. Winter Solstice, 01.01 91 91 91 91 D-92 (04.92), B-92 (02.92) B-92 (02.92) (W27.0)
Chris Carrier 7 Monday, Jan 1 35 28 28 35 28 28 35 28 28 35 28 28 Dec 29-35
Bromberg 7 Monday, Jan 1 28 35 28 28 35 28 28 35 28 28 35 28 Dec 29-35
McClennon 7 Monday, Jan 1 30 30 31 30 30 31 30 30 31 30 30 31 * *Newton 07.1-7, moves following months forward 1
Mills 7 Jan 1 31 30 31 30 31 30 30 31 30 31 30 30 Dec 31

Perpetual calendars[]

Many calendar reforms have offered solutions to make the calendar perpetual. These reforms would make it easy to work out the day of week of a particular date, and would make changing calendars each year unnecessary. There are, roughly speaking, two options to achieve this goal: leap week calendars and intercalary days. Both make it easier to work out the day of week by having exactly 52 weeks in each year. The former add a whole 53rd week every five or six years, the latter have an extra day not belonging to any week and two such in leap years. Proposals mainly differ in their selection of a leap rule, placing of the leap item (usually middle or end of the year), in the start day of the week and year, in the number (12 or 13) and size of months and in connected naming; some are compatible to the week date of ISO 8601.

The World Calendar, favored by the UN in the 1950s, and the International Fixed Calendar, quite popular among economists between the World Wars, are proposals that start each year on a Sunday. The remaining 364 days then form 52 weeks of 7 days. The World Calendar has every third month beginning on the same day of week. In the World Calendar, the 365th and 366th day are considered holidays and named Worlds Day and Leap Year Day. These “off-calendar” days stand outside the seven-day week and caused some religious groups to strongly oppose adoption of The World Calendar. Such concerns helped prevent the World Calendar from being adopted. Supporters of the World Calendar, however, argue that the religious groups’ opposition overlooked every individual’s right to celebrate these holidays as extra days of worship, or Sabbaths. This option, they reason, maintained the seven-day worship cycle for those who share that concern, while allowing benefits of a perpetual calendar to be shared by all.

Leap week calendars add a leap week of seven days to the calendar every five or six years to keep the calendar roughly in step with the tropical year. They have years of either 364 days (52 weeks) or 371 days (53 weeks), thus preserving the 7-day week.

Some calendars have quarters of regularly patterned uneven months e.g. a 35 day (five week) month and a pair of 28 day (four week) months, with a leap week appended to the final month when needed. The Common Civil Calendar and Time calendar has months of 30 and 31 days, but inserts a leap week in the middle of the year, when needed.

The 53-week calendar, used in government and in business for fiscal years, is a variant of this concept. Each year of this calendar can be up to 371 days long.

Still other proposals abandon attempts to make the calendar perpetual, instead opting for eleven 30-day months and one long month at the end of 35 (or 36) days.

13-month calendars[]

Some calendar reformers seek to equalize the length of each month in the year. This is often accomplished by creating a calendar that has 13 months of 4 weeks (28 days) each, making 364 days.

An early 13-month proposal was the 1849 Positivist calendar, created by Auguste Comte. It was based on a 364-day year which included one or two “blank” days. Each of the 13 months had 28 days and exactly four weeks, and each started on a Monday. The International Fixed Calendar is a more modern descendant of this calendar.

Some proposals add one or two days to the calendar each year to account for the annual solar cycle, while others keep these days off the calendar entirely, to make the calendar perpetual.

Around 1930 James A. Colligan invented the Pax Calendar, which avoids off-calendar days by adding a 7-day leap week to the perpetual 364-day year for 71 out of 400 years.

Dividing the years into 52 weeks creates 13 months of 28 days (4 weeks), and 4 quarters of 91 days (13 weeks). This creates months and quarters of fixed, even duration, but not quarters containing a whole number of months.

Lunisolar calendars[]

See also: Lunisolar calendar

Lunisolar calendars usually have 12 or 13 months of 29 or 30 days.

Some propose to improve leap rules of existing calendars, such as the Hebrew calendar. The Rectified Hebrew calendar uses a more accurate leap cycle of 4366 months per 353-year cycle, with 130 leap years per cycle, and a progressively shorter molad interval, intended to replace the 19-year leap cycle and the constant molad interval of the traditional fixed arithmetic Hebrew calendar, respectively.


Calendar proposals that introduce a thirteenth month or change the Julian-Gregorian system of months often also propose new names for these months. New names have also been proposed for days out of the week cycle (e.g. 365th and leap) and weeks out of the month cycle.

Proposals to change the traditional month and weekday names are less frequent. The Gregorian calendar obtains its names mostly from gods of historical religions (e.g. Thursday from Nordic Thor or March from Roman Mars) or leaders of vanished empires (July and August from the first Cæsars), or ordinals that got out of synchronization (September through December, originally seventh through tenth, now ninth through twelfth).

Calendar reformers, therefore, seek to correct what they see as deficiencies by focusing on more homogeneous sets of individuals, who usually share common traits.


Comte’s Positivist calendar, for example, proposed naming the 13 months in his calendar after figures from religion, literature, philosophy and science. Similarly, the Hermetic Lunar Week Calendar uses 12 or 13 lunar months named after 13 pioneers in research on psychoactive plants and chemicals. The Archetypes Calendar names its months after Greek and Roman deities associated with the Sun and the planets. Less imaginatively, the Pontisso Simple Lunisolar Calendar names its months after the letters of the Greek alphabet.

Some have suggested using an existing 13 × 4 naming system, such as playing cards. Thus either months are numbered Ace, Two through Ten, Jack, Queen and King with four weeks each, named after the four suits (♣♠♥♦); or the roles are reversed if the calendar has four quarters with thirteen weeks each. Leap days or weeks are assigned the Joker. This system has internationalisation problems, though, because even where the 52-card deck is known, the order of suits may vary. Also the contemporary roman alphabet has 26 letters, which could be used, together with a further binary indicator, as keys for 52 weeks.

Epoch proposals[]

Year 1 of the Holocene calendar is at 10,000 BC.

See also[]

External links[]

Supplementary External Links[]

Additional references[]