International Fixed Calendar

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 divides the solar year into 13 months of 28 days each. It is therefore a perennial calendar, with every date fixed to the same weekday every year. Though it was never officially adopted in any country, entrepreneur George Eastman adopted it for use in his Eastman Kodak Company, where It also was used 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 (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.[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) and including the mid-year solstice, 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:

Days of the week Leap Day
in June
on leap years,
or Year Day
in December
Sun Mon Tue Wed Thu Fri Sat

Today on this calendar will be 11 December 2019.

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 1January 28
February January 29February 25
March February 26March 25*
April March 26*April 22*
May April 23*May 20*
June May 21*June 17*
June 17 (Leap Day)
Sol June 18July 15
July July 16August 12
August August 13September 9
September September 10October 7
October October 8November 4
November November 5December 2
December December 3December 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 the Rev. Hugh Jones, an American colonist from Maryland writing under the pen name Hirossa Ap-Iccim.[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 (still traditionally used in US, but uncommon in most other countries 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.[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.

  • 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 in 13 months.
  • 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.
  • 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]


  • While each quarter would be equal in length (13 weeks), thirteen is a prime number, thus placing all activities currently done on a quarterly basis out of alignment with the months.
  • Christian, Islamic and Jewish leaders are historically opposed to the calendar, as 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]
  • Birthdays, significant anniversaries, and other holidays would need to be recalculated as a result of a calendar reform, and would always be on the same day of the week. This could be problematic for public holidays that would fall on non-working days under the new system. For example, if a public holiday is celebrated on January 8, then under the International Fixed Calendar that holiday would always fall on a Sunday, which is already a non-working day, so compensatory leave would have to be given each year on January 9, which would essentially change the date of the holiday.
  • A vast amount of administrative data (and the software that manages it) would have to be corrected/adjusted for the new system, potentially having to support both the IFC and the standard local time keeping systems for a period of time.
  • Expiration dates on food items will have to be converted to the new calendar, and checking the dates to make sure the food is fresh could become harder and more confusing as a result, in the first couple years after the switch.

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. Benjamin J. Elton (February 24, 2012). "Calendar Reform and Joseph Herman Hertz". Jewish Telegraphic Agency. Retrieved October 4, 2019.
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