Calendar Wiki

Why is Gregorian calendar reform desirable?[]

Before considering the prospects for calendar reform we should be clear about why reform of the calendar is desirable.

Calendar reform means improving, or replacing, the Common Era Calendar which, since early in the 20th Century, has been the standard calendar in use in most countries in the world.

Benefits of the Gregorian calendar[]

Reasons for retaining this calendar are:

  1. The calendar year presently stays reasonably well in sync with the seasonal year: the vernal equinox or northward equinox occurs during a 51-hour range that is spread over March 19, 20 and 21.
  2. The Gregorian reform also improved the parallel lunisolar cycle that is employed for the Easter computus. Its mean year is exactly the same as the solar calendar mean year, and its mean month is well suited to the present era mean lunation interval.
  3. Most nations and cultures are familiar with the Gregorian calendar.
  4. The rules of the calendar are already embedded in innumerable computer programs (although not necessarily correctly).
  5. It maintains the traditional uninterrupted seven-day weekly cycle, which is important to several major religious groups.
  6. It may be difficult to promulgate changes to the calendar because all countries that use it would need to agree to make a change. The Gregorian calendar took nearly 350 years to be adopted by most of the countries that previously used the Julian calendar, and the Julian calendar continues to be used in several countries, particularly by the Orthodox Church. On the other hand, today's rapid internet communication makes it unlikely that the next calendar reform will take long to spread, once it gets going.

Possible benefits of a calendar reform[]

Reasons for modifying or replacing this calendar are:

  1. The conventional 7‑day‑week cycle does not fit exactly into a Gregorian calendar year (there are always one or, for leap years, two days left over). This means that it is difficult to know which day of the week a Gregorian date falls on. The majority of Gregorian calendar reform proposals are designed to make the new calendar perpetual, by starting every year on a specific weekday. This enhancement should be accomplished, however, without any disruption of the traditional 7-day week, for example by ensuring that both common and leap years contain a whole number of weeks, because calendars that introduce a null weekday in common years and an extra null weekday in leap years have already been rejected for that reason by the League of Nations in 1937 (Thirteen Month Calendar) and the United Nations in 1955 (World Calendar).
  2. The structure of the months seems irregular, with month lengths ranging arbitrarily from 28 days to 31 days.
    1. The irregularity of the Gregorian calendar structure and the varying weekday for New Year Day makes it impossible to schedule events to occur on certain days of the week and then reuse that unmodified schedule from year to year, whereas this would be easily done using a perpetual calendar.
    2. That irregularity also makes it very difficult to design schedules which can be used in any quarter/trimester (of three months), term/quadmester (of four months) or semester (of six months).
  3. The Gregorian leap year rule involves multiple steps, and does not spread the leap years as smoothly as possible, nor are the leap years symmetrically distributed: an extra day is added at the end of the second month in years whose number is divisible by 4, except in years whose number is divisible by 100, unless the year number is divisible by 400.
  4. The Gregorian calendar mean year (365+97/400 days = 365 days 5 hours 49 minutes 12 seconds) is nearly 12 seconds too long relative to the target mean northward equinoctial year. Due to progressive tidal slowing of the Earth rotation rate, it would be better for the calendar to have a mean year that is intentionally slightly too short.
    1. The Gregorian leap year rule causes the timing of the northward equinox to vary over a 51-hour range, which could be reduced to less than 24 hours if a smoothly spread leap year rule were adopted.
  5. Despite the existence of a proposed standard way of writing Gregorian dates (the ISO 8601 date format), such dates are currently expressed mainly either as month-day-year (in the U.S.) or year-month-day (in Canada) or day-month-year (in Europe and most of the rest of the world). This creates confusion for people in one part of the world reading dates written by and for people in another part of the world.
  6. The months of the Gregorian calendar, although called months from ‘moon’, have no relation to the lunar cycles. The sequence of months and the sequence of lunations are completely unrelated, so a new moon or a full moon can occur on any day of the Gregorian month. (On the other hand, the lunisolar Gregorian Easter computus is a good approximation to the mean lunar cycle, and switching to a lunisolar calendar for general use would not be a calendar reform but rather a calendar replacement.)
  7. The intercalary (leap) day is inserted at the end of the second month instead of at the end of the year, which adds complexity to various date calculations. In particular, the number of days between a particular date in January or February and a particular date after the end of February is not constant.

Calendar reform seeks an improved calendar, to the extent that is possible, while avoiding as many of the deficiencies mentioned above as possible.

Transitional considerations[]

The Gregorian calendar reform was promulgated by the Roman Catholic Church. In the 20th century the League of Nations and the United Nations both had a stab at calendar reform, but failed. Later in the 20th century, the International Organization for Standardization introduced the ISO leap week calendar, which counts years, weeks and days, and runs parallel to the Gregorian calendar. What other organization might command sufficient authority to introduce an improved calendar reform?

Any proposal for calendar reform must consider how the transition from the old to the new is to be managed. Some considerations are:

  • How many attributes do the old and the new calendars have in common, such as familiar weekday and month names and parallel year numbering?
  • What, if anything, about the new calendar will motivate people to use it? In all likelihood, only a perpetual calendar reform can offer a compelling reason to actually motivate its use, because very few people care about issues such as the accuracy of astronomical approximations or the advantages of smoothly and symmetrically spread leap year intervals.
  • How easy will it be for people to inter-convert old and new calendar dates? This is an important consideration, although it would be a simple matter to publish date conversion lists, or to show both old and new calendar dates on printed calendars during the transition, or to provide internet date conversion resources or personal computer date conversion programs.
  • What is to be done with the legacy software which was programmed to handle old calendar dates? How easy will it be for programmers to modify computer programs to handle new calendar dates? Are the necessary arithmetic algorithms published and in the public domain for royalty-free use? Computer software revisions will be easiest in programs that call a unified date input function for all date input operations and a unified date output function for all date output operations, and where dates are stored internally as ordinal serial numbers rather than as separated text dates, thus avoiding any need to convert old date records. In such cases it is possible to allow multiple users sharing the same system to be simultaneously active using their choice of old or new calendar, switching on the fly if they so choose. Program revisions will be more difficult if the text space required to represent dates on screen displays and report printouts is not the same as is presently used for Gregorian dates. A major first step will be the availability of the new calendar as a system calendar choice in major operating systems such as Windows, Mac OS and Linux, either implemented by the operating system developer or by third parties.
  • For how long should the old and the new calendars be maintained concurrently before the old is definitely seen to be a thing of the past? Keep in mind that although the Gregorian reform was introduced more than 4 centuries ago, use of the old Julian calendar continues in the world of historians and the Orthodox Church.