International Fixed Calendar

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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 designed by Moses B. Cotsworth who presented it in 1923 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. Though it was never officially adopted in any country, it was the official calendar of the Eastman Kodak Company from 1928 to 1989.[1]



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 (and so is a solar calendar).

The months have the same names as those of 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.

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.

Because 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:

The 13 months and extra days occur on the following Gregorian dates:

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


The International Fixed Calendar League was founded in 1923 by Moses B. Cotsworth, with offices in London and later in Rochester, New York. It ceased activities in the 1930s.

George Eastman of the Eastman Kodak Company was a fervent supporter of the IFC, and instituted its use at Kodak in 1928, where it remained in use until 1989.[1]

In recent years, there have been attempts to revive the plan.[citation needed]

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