The International Fixed Calendar (also known as the IFC, Cotsworth plan, the Cotsworth calendar and the Eastman plan) is a proposed calendar reform designed by Moses B. Cotsworth, first presented in 1902.[1] The solar calendar divides the year into 13 months of 28 days each. A type of perennial calendar, every date is fixed to the same weekday every year. Though it was never officially adopted at the country level, the entrepreneur George Eastman instituted its use at the Eastman Kodak Company in 1928, where it was used until 1989.[2] While it is sometimes described as the 13-month calendar or the equal-month calendar, various alternative calendar designs share these features.


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 (after December 28, i.e. equal December 31 Gregorian), 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.[a] 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) and including the mid-year solstice, the name of the new month was chosen in homage to the sun.[3]

Leap years in the International Fixed Calendar contain 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 years 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:

Days of the week Leap Day
in June
on leap years,
or Year Day
in December
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 29

The date for today using this calendar is 17 August 2023.

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:

Fixed calendar
Matching dates on the Gregorian calendar
Starts on fixed day 1 Ends on fixed day 28 (or 29)
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*
June 17 (Leap Day)
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
December 31 (Year Day)

*These Gregorian dates between March and June are a day earlier in a Gregorian leap year. March in the Fixed Calendar always has a fixed number of days (28), and includes the Gregorian February 29 (on Gregorian leap years).


Lunisolar calendars, with fixed weekdays, existed in many ancient cultures, with certain holidays always falling on the same dates of the month and days of the week.

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 Reverend Hugh Jones, an American colonist from Maryland writing under the pen name Hirossa Ap-Iccim.[4] 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.[5] 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 (still traditionally used in US, but uncommon in Europe and in the ISO (International Organization for Standardization) week standard, starting their weeks on Monday), 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.[6] Sir Sandford Fleming, the inventor and driving force behind worldwide adoption of standard time, became the first president of the IFCL.[7] 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.[8]


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

The main and undeniable function of the calendar is to measure the passage of time. This feature is mainly used for current use and only occasionally for historical events. Hence the conclusion that its everyday functionality should be considered basic. Besides, if it is a good "measuring instrument" that meets the appropriate standards, then finding historical dates on its scale will be intuitive and easy, and you only need to enter them once.

Measuring in a methodologically sound and user-friendly way – implies some basic expectations about the units of time that will be used:

1. They should be equal, otherwise the accuracy of the "meter" indications, and certainly the convenience of its use, becomes questionable. This postulate is satisfied because all months except the last one have the same length, and years are accurate to one day.

2. The inaccuracy of the "meter" indications in relation to astronomical time - should not be greater than the smallest time unit it shows. This postulate gives him an advantage over projects with an added intercalary week in irregular years.


See also



  1. ^ See the table in Cotsworth 1904, p. i


  1. ^ Cotsworth 1904.
  2. ^ Exhibit at George Eastman House, viewed June 2008
  3. ^ 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.
  4. ^ Hirossa Ap-Iccim, "An Essay on the British Computation of Time, Coins, Weights, and Measures" The Gentleman's Magazine, 15 (1745): 377-379
  5. ^ Cotsworth 1904, p. i.
  6. ^ Duncan Steel, Marking Time: The Epic Quest to Invent the Perfect Calendar (New York: John Wiley & Sons, 2000), page 309
  7. ^ Moses Bruine Cotsworth, Calendar Reform (London: The International Fixed Calendar League, 1927), Preface.
  8. ^ Journal of Calendar Reform volume 16, number 4 (1944): 165-66
  9. ^ Bull, Jonathan R.; Rowland, Simon P.; Scherwitzl, Elina Berglund; Scherwitzl, Raoul; Danielsson, Kristina Gemzell; Harper, Joyce (August 27, 2019). "Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles". npj Digital Medicine. 2 (1): 83. doi:10.1038/s41746-019-0152-7. ISSN 2398-6352. PMC 6710244. PMID 31482137.
  10. ^ "The Death and Life of the 13-Month Calendar". December 11, 2014. Retrieved June 2, 2022.
  11. ^ Stockbridge, Frank Parker (June 1929). "New calendar by 1933 - Eastman". Popular Science Monthly. No. 32. pp. 131–133.
  12. ^ Eastman, George (May 1926). "The importance of calendar reform to the world of business". The Nation's Business. pp. 42, 46.
  13. ^ Benjamin J. Elton (February 24, 2012). "Calendar Reform and Joseph Herman Hertz". Jewish Telegraphic Agency. Retrieved October 4, 2019.