A fixed, perpetual calendar system with 13 equal months of 28 days each. Every date falls on the same weekday, every year.
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The 13 month calendar, also known as the International Fixed Calendar, is a solar calendar reform proposal that divides the year into 13 months of exactly 28 days each. This creates a perpetual calendar where every date always falls on the same day of the week, year after year.
Originally proposed by Moses B. Cotsworth in 1902 and later championed by George Eastman (founder of Kodak) in the 1920s, this calendar system was seriously considered by the League of Nations in 1928 as a potential worldwide standard. While it was never officially adopted, it remains one of the most practical calendar reform proposals ever created.
Each of the 13 months contains exactly 28 days (4 weeks). This means 13 x 28 = 364 days, with one extra "Year Day" added at the end of the year.
Every month starts on Sunday and ends on Saturday. January 1st is always a Sunday. The 15th is always a Sunday. Dates never shift between years.
The additional month, called "Sol," is inserted between June and July. It's named after the sun and falls during the summer solstice period.
"Year Day" (December 29th in Fixed) exists outside any month or week. In leap years, an extra "Leap Day" is added after June 28th.
The concept of a 13-month calendar has ancient roots, with many early civilizations using lunar calendars with 13 months. The modern International Fixed Calendar was developed by Moses B. Cotsworth, a British accountant who sought to create a more rational calendar for business and commerce.
George Eastman, the founder of Kodak, became the calendar's most prominent advocate. He used it internally at Kodak from 1928 to 1989 and lobbied extensively for its worldwide adoption. The League of Nations considered it in 1928, but religious objections to the "blank day" concept prevented its adoption.
Start with the definition, then compare it to the calendar you use today. These guides explain the system without the noise.
The main opposition came from religious groups who objected to the "Year Day" concept, which would break the continuous seven-day week cycle that has religious significance in Judaism, Christianity, and Islam.
In the proposed system, most holidays would shift to fall on the same date and weekday every year. For example, if a holiday was designated as "the third Monday of Sol," it would always be Sol 15th.
Use our date converter tool to instantly convert any date between the Gregorian calendar and the 13 month fixed calendar.
While no country officially uses it, some organizations and individuals use it for personal planning. It remains a popular topic among calendar reform enthusiasts.
Explore the 13 Month Calendar
Use the interactive calendar above to see how dates map between systems, or discover your true birthday.
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