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International Fixed Calendar

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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 International Fixed 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 to 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, making a long weekend. As a result, a particular day usually has a different day of the week in the IFC than in all traditional calendars that contain a seven-day week. The IFC is, however, almost compatible with the World Calendar in this regard, because it also starts Sunday and has the extra day at the end of the year and the leap day in the middle, except IFC leaps on Gregorian June 17 and TWC leaps two weeks later on July 1. Since this break of the ancient week cycle has been a major concern raised against its adoption, various leap week calendars have been proposed as a solution.

Common layout of all months
Days of the week
Sun Mon Tue Wed Thu Fri Sat Hol
01 02 03 04 05 06 07 Leap Day,
Year Day
08 09 10 11 12 13 14
15 16 17 18 19 20 21
22 23 24 25 26 27 28 X*

* The two special dates have been recorded as either the 29th day of the month ending or the 0th day of the month beginning, or, more correctly, as outside any month and week with no ordinal number.

The date for today, 15 June 2024, using this calendar is Friday, 27 June 2024.

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

IFC Matching dates on the Gregorian calendar
Starts on fixed day 1 Ends on fixed day 28
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

* In a leap year, these Gregorian dates between March and June are a day earlier. March in the Fixed Calendar always has a fixed number of days (28), and includes an eventual Gregorian February 29. The rule for finding leap years is the same in both calendars.


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 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 subdivision of the year is very regular and systematic:
    • Each month has exactly 4 weeks (28 days).
    • Every day of the month falls on the same weekday in each month (e.g. the 17th always falls on a Tuesday).
    • Every year has exactly 52 weeks divided among 13 months.
    • Every year has 4 equal-length quarters of 91 days, each 13 weeks or 3+14 months long.
  • 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.
  • Movable holidays celebrated on the nth certain weekday of a month, such as U.S. Thanksgiving 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.
  • Although the average menstrual cycle of women is often described as 28 days, there is variation and debate about this in the literature.[9] Still, a consistent number of days in a month might prove beneficial for tracking periods and fertility with more ease.
    • The first mention of this argument can be found in September 1927 issue of The Outlook briefly mentioned an advantage to women without detail. A 2014 article in Bloomberg that explores that previous article cites an unnamed woman the author personally knew who sarcastically disagreed with such an advantage.[10] Overall opinions of women on this calendar change as related to the menstrual cycle haven't been studied.
  • 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.[11][12]


  • Since its 13 months are not easily grouped into four season-like quarters, this calendar obfuscates how Earth circles the Sun from a rational human-centric perspective. Solstices and equinoxes are on fixed dates but those are not as similar as in the Gregorian calendar, i.e. 20th to 23rd day of the 3rd, 6th, 9th and 12th month. Likewise, the start dates of the astrological signs of the zodiac are more arbitrarily placed, while the underlying constellations are now disconnected in any calendar.
  • 13-week quarters would start on 1 January, 8 April, 15 Sol and 22 September, so all activities currently done on a quarterly basis would be placed out of alignment with the months. Q4 and, in leap years, Q2 would be one day longer.
  • Some Jewish and Christian leaders denounced the calendar as their tradition of worshipping 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.[13]
  • The calendar is inconsistent with ISO 8601 regarding the first weekday of the week (Sunday vs. Monday), meaning major parts of the world would have to change their first weekday of the week.
  • Birthdays, significant anniversaries, and other holidays would always be on the same day of the week. This could be seen as problematic for public holidays that would fall on non-working days under the new system: For example, New Year's Day, International Labor Day and All Hallows are all celebrated on the first day of their respective month, which under the International Fixed Calendar would always be a Sunday, a non-working day, so people might want to be compensated by another day off.
  • For any holidays or recurring events that take place on the 29th, 30th or 31st day of a month, a new date would have to be determined. This could be the last, 28th day or the first day of the following month. Alternatively, all anniversaries could be celebrated on the date they originally occurred on if the IFC had been in use back then already, e.g., the US Independence Day of Gregorian 4th of July in 1776 would be on Thursday, Sol 19.

See also[edit]



  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". Bloomberg.com. 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.


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