International Fixed Calendar

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The International Fixed calendar (also known as the Cotsworth plan, the Eastman plan, the 13 Month calendar or the Equal Month calendar) is a solar calendar proposal for calendar reform designed by Moses B. Cotsworth, who presented it in 1902.[1] It provides for a year of 13 months of 28 days each. It is therefore a perennial calendar, with every date fixed always on the same weekday. Though it was never officially adopted in any country, it was the official calendar of the Eastman Kodak Company from 1928 to 1989.[2]


The calendar year has 13 months with 28 days each, divided into exactly 4 weeks (13 × 28 = 364). An extra day added as a holiday at the end of the year (December 29), sometimes called "Year Day", does not belong to any week and brings the total to 365 days. Each year coincides with the corresponding Gregorian year, so January 1 in the Cotsworth calendar always falls on Gregorian January 1.[3] Twelve months are named and ordered the same as those of the Gregorian calendar, except that the extra month is inserted between June and July, and called Sol. Situated in mid-summer (from the point of view of its Northern Hemisphere authors), the name of the new month was chosen in homage to the sun.[4]

Leap year in the International Fixed Calendar contains 366 days, and its occurrence follows the Gregorian rule. There is a leap year in every year whose number is divisible by 4, but not if the year number is divisible by 100, unless it is also divisible by 400. So although the year 2000 was a leap year, the years 1700, 1800, and 1900 were common years. The International Fixed Calendar inserts the extra day in leap year as June 29 - between Saturday June 28 and Sunday Sol 1.

Each month begins on a Sunday, and ends on a Saturday; consequently, every year begins on Sunday.

Neither Year Day nor Leap Day are considered to be part of any week; they are preceded by a Saturday and are followed by a Sunday.

All the months look like this:

Sun Mon Tue Wed Thu Fri Sat
1 2 3 4 5 6 7
8 9 10 11 12 13 14
15 16 17 18 19 20 21
22 23 24 25 26 27 28

The following shows how the 13 months and extra days of the International Fixed Calendar occur in relation to the dates of the Gregorian calendar:

Month Starts Ends
January January 1 January 28
February January 29 February 25
March February 26 March 25*
April March 26* April 22*
May April 23* May 20*
June May 21* June 17*
Leap Day June 17
Sol June 18 July 15
July July 16 August 12
August August 13 September 9
September September 10 October 7
October October 8 November 4
November November 5 December 2
December December 3 December 30
Year Day December 31

*These dates are a day earlier in a leap year.


The simple idea of a 13-month perennial calendar has been around since at least the middle of the 18th century. Versions of the idea differ mainly on how the months are named, and the treatment of the extra day in leap year.

The Georgian calendar was proposed in 1745 by an American Colonist from Maryland writing under the pen name, Hirossa Ap-Iccim (=Rev. Hugh Jones).[5] The author named the plan, and the thirteenth month, after King George II of Great Britain. The 365th day each year was to be set aside as Christmas. The treatment of leap year varied from the Gregorian rule, however; and the year would begin closer to the winter solstice. In a later version of the plan, published in 1753, the 13 months were all renamed for Christian saints.

In 1849 the French philosopher Auguste Comte (1798–1857) proposed the 13-month Positivist Calendar, naming the months: Moses, Homer, Aristotle, Archimedes, Caesar, St. Paul, Charlemagne, Dante, Gutenberg, Shakespeare, Descartes, Frederic and Bichat. The days of the year were likewise dedicated to "saints" in the Positivist Religion of Humanity. Positivist weeks, months, and years begin with Monday instead of Sunday. Comte also reset the year number, beginning the era of his calendar (year 1) with the Gregorian year 1789. For the extra days of the year not belonging to any week or month, Comte followed the pattern of Ap-Iccim (Jones), ending each year with a festival on the 365th day, followed by a subsequent feast day occurring only in leap years.

Whether Moses Cotsworth was familiar with the 13-month plans that preceded his International Fixed Calendar is not known. He did follow Ap-Iccim (Jones) in designating the 365th day of the year as Christmas. His suggestion was that this last day of the year should be designated a Sunday, and hence, because the following day would be New Year's Day and a Sunday also, he called it a Double Sunday.[6] Since Cotsworth's goal was a simplified, more "rational" calendar for business and industry, he would carry over all the features of the Gregorian calendar consistent with this goal, including the traditional month names, the week beginning on Sunday, and the Gregorian leap-year rule.

To promote Cotsworth's calendar reform The International Fixed Calendar League was founded in 1923, just after the plan was selected by the League of Nations as the best of 130 calendar proposals put forward.[7] Sir Sandford Fleming, the inventor and driving force behind worldwide adoption of standard time, became the first president of the IFCL.[8] The League opened offices in London and later in Rochester, New York. George Eastman, of the Eastman Kodak Company, became a fervent supporter of the IFC, and instituted its use at Kodak. The International Fixed Calendar League ceased operations shortly after the calendar plan failed to win final approval of the League of Nations in 1937.[9]


The several advantages of The International Fixed Calendar are mainly related to its organization.

  • Every year has exactly 52 weeks divided in 13 months.
  • Each month has exactly 28 days divided in 4 weeks.
  • The calendar is the same every year (perennial), unlike the annual Gregorian calendar, which differs from year to year. Hence, scheduling is easier for institutions and industries with extended production cycles.
  • Every day of the month falls on the same weekday in each month—the 17th always falls on a Tuesday, for example.
  • Moveable holidays celebrated on the nth weekday of a month, such as Labor Day, would be able to have a fixed date while keeping their traditional weekday.
  • Statistical comparisons by months are more accurate, since all months contain exactly the same number of business days and weekends; likewise for comparisons by 13-week quarters.
  • Supporters of the International Fixed Calendar have argued that thirteen equal divisions of the year are superior to twelve unequal divisions in terms of monthly cash flow in the economy.[10]


  • For the superstitious, a disadvantage to this format is that every month includes a Friday the 13th, and this date occurs thirteen times every year. This is readily solved by making the 1st a Monday.
  • Thirteen, being prime, is not evenly divisible, putting all activities currently done on a quarterly basis out of alignment with the months; each quarter would be 13 weeks instead.
  • Some Jewish, Christian, and Islamic groups have been historically opposed to the calendar because their tradition of worshiping every seventh day would result in either the day of the week of worship changing from year to year or eight days passing when Year Day or Leap Day occurs.[11] Others have contended that Year Day and Leap Day could be counted as additional days of worship.
  • Birthdays and other significant anniversaries may be recalculated as a result of a calendar reform and would always be on the same day of the week. If the recalculated day is a weekend, it cannot be designated a holiday and causes difficulty in celebrating this day. This is readily solved by making the following weekday a holiday.
  • Sunday as the 'first day of the week' feels awkward for the nations that typically show calendars with Monday as the first day of the week. As such, they would be likely to push back against what feels like the 'breaking up' of the two weekend days, leading to a lowered willingness to adopt this calendar.

See also


  1. Moses B. Cotsworth, The rational almanac: tracing the evolution of modern almanacs from ancient ideas of time, and suggesting improvements (Acomb, England:Cotsworth, 1905)
  2. Exhibit at George Eastman House, viewed June 2008
  3. See the table in Cotsworth, Rational Almanac, p. i.
  4. Cotsworth suggested "Mid" as an alternative name. See his address in Royal Society of Canada, Proceedings and Transactions of the Royal Society of Canada, 3d series, vol. II (Ottawa: James Hope & Son, 1908), pp. 211-41 at 231.
  5. Hirossa Ap-Iccim, "An Essay on the British Computation of Time, Coins, Weights, and Measures" The Gentleman’s Magazine, 15 (1745): 377-379
  6. Cotsworth, The Rational Almanac, p. i.
  7. Duncan Steel, Marking Time: The Epic Quest to Invent the Perfect Calendar (New York: John Wiley & Sons, 2000), p. 309.
  8. Moses Bruine Cotsworth, Calendar Reform (London: The International Fixed Calendar League, 1927), Preface.
  9. See Journal of Calendar Reform vol. 16, no. 4 (1944): 165-66
  10. See Frank Parker Stockbridge, "New Calendar by 1933—Eastman," Popular Science Monthly (June 1929): 32, 131-33; and George Eastman, "The Importance of Calendar Reform to the World of Business," Nation's Business (May 1926): p. 42, 46.
  11. Joseph Hertz, Calendar Reform

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