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The International Fixed calendar (also known as the International Perpetual calendar, the Cotsworth plan, the Eastman plan, the 13 Month calendar or the Equal Month calendar) is a proposal for calendar reform providing for a year of 13 months of 28 days each, with one day at the end of each year belonging to no month or week.


The calendar year has 13 months each with 28 days plus an extra day at the end of the year not belonging to any month. Each year coincides with the corresponding Gregorian year.

The months are named the same as for the Gregorian calendar except that a month called Sol is inserted between June and July.

In leap years, a leap day, also belonging to no month is inserted after June and before the new month. Common years are 365 days long; leap years are 366 days long.

The first day of each year, January 1, is deemed a Sunday and every subsequent day that belongs to a month is deemed to be in the conventional 7-day week (the days of which go, in order, Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, and Saturday, followed by Sunday of the next week).

Days that do not belong to a month are deemed to be outside the week and always occur between a day deemed Saturday and a day deemed to be Sunday.

Since each month consists of exactly four weeks, the first day of each month and every seventh day after that for the rest of the month is deemed to be a Sunday, the second day of each month and every seventh day after that for the rest of the month is deemed to be a Monday, and so on. Therefore, each month begins on a Sunday and ends on a Saturday, just like each conventional week.

This causes all months to look like this:

Week 1 2 3 4 5 6 7
Sun Mon Tue Wed Thu Fri Sat
1 01 02 03 04 05 06 07
2 08 09 10 11 12 13 14
3 15 16 17 18 19 20 21
4 22 23 24 25 26 27 28

Gregorian dates of the 13 months and extra days
Month Start End
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 International Fixed Calendar League, with offices in London and in Rochester, New York, ceased activities in the 1930s. In recent years, there have been attempts to revive the plan.

The International Perpetual calendar is based on the Positivist Calendar published in 1849 by French philosopher Auguste Comte (1798-1857). Comte based his calendar on Polynesian calendars. The main difference between the International Perpetual calendar and the Positivist calendar is the names Comte gave to months and days. Positivist weeks, and Positivist months, begin with Monday instead of Sunday. Whereas the Positivist and Sol Calendars place the leap day at the end of the leap year, the International Fixed Calendar and the World Calendar both place it after June.

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