Calendar reform


Calendar reform or calendrical reform is any significant revision of a calendar system. The term sometimes is used instead for a proposal to switch to a different calendar design.

Principles

The prime objective of a calendar is to unambiguously identify any day in past, present and future by a specific date in order to record or organize social, religious, commercial or administrative events. Recurring periods that contain multiple days, such as weeks, months, and years, are secondary, convenient features of a calendar. Most cultures adopt a primary dating system, but different cultures have always needed to align multiple calendars with each other, either because they coexisted in the same space or had established trading relations.
Once specified, a calendar design cannot be altered without becoming a new design. If a proposed design is sufficiently close to the legacy one, i.e. compatible with it, a reform of the local calendar system is possible without disruption. Examples of this include the changes to the Chinese calendar due to problems between regions in China and practical changes in religious calendars such as the Islamic calendar, where the start of the month is now derived from astronomical data rather than sightings by religious leaders.
Some design changes, however, will yield date identifiers different from the previous design for some days, often in the distant past or future.
The calendar system must clarify whether dates are changed to the new design retroactively or whether the design in use then and there shall be respected. Calendar schisms happen if not all cultures that adopted a common calendar system before perform a calendar reform at the same time. If date identifiers are similar but different, confusion and mistakes are unavoidable.
Most calendars have several rules which could be altered by reform:
  • Whether and how days are grouped into subdivisions such as months and weeks, and days outside those subdivisions, if any.
  • Which years are leap years and common years and how they differ.
  • Numbering of years, selection of the epoch, and the issue of year zero.
  • Start of the year.
  • If a week is retained, the start, length, and names of its days.
  • Start of the day.
  • If months are retained, number, lengths, and names of months.
  • Special days and periods.
  • Alignment with social cycles.
  • Alignment with astronomical cycles.
  • Alignment with biological cycles.
  • Literal notation of dates.

    Historical reforms

Historically, most calendar reforms have been made in order to synchronize the calendar with the astronomical year 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 altered to the Gregorian calendar.
The fundamental problems of the calendar are that the astronomical year has neither a whole number of days nor a whole number of lunar months; neither does the lunar month have a whole number of days: in each case there are fractions "left over". Such remainders could accumulate from one period to the next, thereby driving the cycles out of synchronization.
A typical solution to force synchronization is intercalation. This means occasionally adding an extra day, week, or month into the cycle. An alternative approach is to ignore the mismatch and simply let the cycles continue to drift apart. The general approaches include:
  • The lunar calendar, which fits days into the cycle of lunar months, adding an extra day when needed, while ignoring the annual solar cycle of the seasons.
  • The solar calendar, which fits artificial months into the year, adding an extra day into one month when needed, while ignoring the lunar cycle of new/full moons.
  • The lunisolar calendar, which keeps both the lunar and solar cycles, adding an extra month into the year when needed.
An obvious disadvantage of the lunisolar method of inserting a whole extra month is the large irregularity of the length of the year from one to the next. The simplicity of a lunar calendar has always been outweighed by its inability to track the seasons, and a solar calendar is used in conjunction to remedy this defect. Identifying the lunar cycle month requires straightforward observation of the Moon on a clear night. However, identifying seasonal cycles requires much more methodical observation of stars or a device to track solar day-to-day progression, such as that established at places like Stonehenge. After centuries of empirical observations, the theoretical aspects of calendar construction could become more refined, enabling predictions that identified the need for reform.

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 completely divorced it from the solar year.
Another reform was performed in Seljuk Persia by Omar Khayyam and others, developing the precisely computed Jalali calendar.

Julian and Gregorian reforms

When Julius Caesar took power in Rome, the Roman calendar had ceased to reflect the year accurately.
The Julian reform made 46 BC 445 days long and replaced the intercalary month with an intercalary day to be inserted within February every four years. This produced a noticeably more accurate calendar, but it had an average year length of 365 days and six hours, which had the effect of adding about three-quarters of an hour every four years. The effect accumulated from inception in 45 BC until by the 16th century the northward equinox was falling on March 10 or 11.
Under Pope Gregory XIII, the leap year rule was altered: only centennial years evenly divisible by 400 are leap years. Thus, the years 1600, 2000, 2400, and 2800 are leap years, while 1700, 1800, 1900, 2100, 2200, 2300, 2500, 2600, 2700, 2900, and 3000 are common years despite being divisible by 4. This rule makes the mean year 365.2425 days long. While this does not synchronize the years entirely, it would require a few thousand years to accumulate a day.
So that the northward equinox would have the same date in the new Gregorian calendar as it had when the Council of Nicaea made recommendations in AD 325, ten days were dropped so that became in 1582. This reform took a few centuries to spread through the nations that used the Julian calendar, although the Russian church year still uses the Julian calendar. Those nations that adopted this calendar on or after 1700, had to drop more than ten days: Great Britain, for instance, dropped eleven.
In 1923, Milutin Milanković proposed to a synod of some Eastern Orthodox Churches at Constantinople that only centennial years that leave a remainder of 200 or 600 upon division by 900 would be leap years, decreasing the average year length to 365.24 days: these remainders were chosen to delay as much as possible the first year that this calendar would disagree with the Gregorian calendar, which occurs in until 2800. It was adopted by some Eastern Orthodox Churches, under the names Revised Julian calendar or New calendar, but was rejected by others.

Proposals

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 from traditional conceptions in many cultures.
Since the papal reform in 1582, several proposals have been offered to make the Gregorian calendar more useful or regular. Very few reforms have gained official acceptance. The rather different decimal French Republican Calendar was one such official reform, but was abolished twelve years later by Napoleon. After World War II, the newly formed United Nations continued efforts of its predecessor, the League of Nations, to establish the proposed World Calendar but postponed the issue after a veto from the government of the United States, which was mainly based upon concerns of religious groups about the proposed days that would be outside the seven-day week cycle and thus disrupt having a sabbath every seven days. Independently the World Council of Churches still tries to find a common rule for the date of Easter, which might be eased by a new common calendar.
Reformers cite several problems with the Gregorian calendar:
  • It is not perennial. 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, and so some people rely on mnemonics to remember the lengths of months.
  • The year's four quarters are not equal. Business quarters that are equal would make accounting easier.
  • Its epoch, i.e. start of the year count, is religious. The same applies to month and weekday names in many languages.
  • Each month has no connection with the lunar phases.
  • Solstices and equinoxes do not coincide with either the beginning of the Gregorian months or the midpoint of the months.
  • The calendar does not have a year zero: the year after 1 BC was 1 AD, with nothing in between them.
It is hard or even impossible to solve all these issues in just one calendar.
Most plans evolve around the solar year of a 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.