Calendar reform

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 (e.g. secular and religious groups with different demands) or had established trading relations.

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.

(In some physical circumstances the rotations and revolutions of a planet and its satellite(s) can be phase-locked – for example the same side of the moon always faces us – but this has not operated to lock together the lengths of the Earth's year, day and month.)

The general approaches include: 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.

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.

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.

Another reform was performed in Seljuk Persia by Omar Khayyam and others, developing the precisely computed Jalali calendar.

When Julius Caesar took power in Rome, the Roman calendar had ceased to reflect the year accurately.

The effect accumulated from inception in 45 BC until by the 16th century the northward equinox was falling on March 10 or 11.

There is also an international standard describing the calendar, ISO 8601, with some differences from traditional conceptions in many cultures.

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 ("blank days") and thus disrupt having a sabbath every seven days.

There are, roughly speaking, two options to achieve this goal: leap week calendars and intercalary days.

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.

These "off-calendar" days stand outside the seven-day week and caused some religious groups to strongly oppose adoption of the World Calendar.

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.

[6] The lengths of the months inherited from the old Roman calendar as reformed by Julius Caesar do not follow any apparent logic systematically.

The Positivist calendar (1849), created by Auguste Comte, was based on a 364-day year which included one or two "blank" days.

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 Caesars), or ordinals that got out of synchronization (September through December, originally seventh through tenth, now ninth through twelfth).