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.
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 try our conversion tools.
Convert Dates Learn the LogicExploring a more natural way to measure time
The 13-Month Calendar project exists to educate, inspire, and provide practical tools for understanding alternative timekeeping systems. We believe that how we measure time profoundly affects how we experience life, plan our activities, and connect with natural cycles.
Our goal is to make the International Fixed Calendar accessible to everyone through interactive visualization tools, educational content, and practical conversion utilities. Whether you're a calendar reform enthusiast, a history buff, or simply curious about alternative ways of organizing time, we're here to help you explore.
View any date in both Gregorian and 13-month formats with our side-by-side calendar display. See how dates map between the two systems.
Track moon phases alongside calendar dates. The 28-day month closely aligns with the lunar cycle, making moon tracking intuitive.
View sun angle, daylight changes, seasons, and Earth's orbital position. Understand how celestial events connect to our calendar.
Convert any date between Gregorian and 13-month calendars instantly. Find your birthday, anniversaries, or any important date.
The International Fixed Calendar (also known as the Cotsworth Plan or the Eastman Plan) was developed by Moses B. Cotsworth, a British accountant, in the early 20th century. The system divides the year into 13 months of exactly 28 days each, totaling 364 days. The 365th day is designated as "Year Day," a standalone holiday between years.
This calendar gained significant support from George Eastman, founder of Kodak, who implemented it for his company's internal operations from 1928 to 1989. The League of Nations seriously considered adopting it as a world standard in the 1930s, though religious objections ultimately prevented its global adoption.
We believe in making complex systems understandable and accessible to everyone.
Our tools are free to use and our approach is transparent and community-focused.
We encourage questioning established systems and exploring alternative possibilities.
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Ready to see the world through a different lens? Try our interactive calendar or convert your important dates to the 13-month system.
View Calendar Convert DatesLast updated: January 2025
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Last updated: January 2025
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Everything you need to know about the 13-month calendar system
The 13-month calendar (also known as the International Fixed Calendar) is an alternative calendar system that divides the year into 13 months of exactly 28 days each. This creates 364 days, with the 365th day designated as 'Year Day' - a standalone holiday that doesn't belong to any month or week.
13 months of 28 days provides several advantages: every month is identical (4 complete weeks), every date falls on the same weekday every year, and the 28-day month closely aligns with the lunar cycle (~29.5 days). This makes planning, scheduling, and understanding time patterns much simpler.
In the International Fixed Calendar, the 13th month is called 'Sol' (after the sun). It is inserted between June and July.
Year Day is the 365th day of the year (December 31st in Gregorian terms). It's a standalone day that doesn't belong to any month or week - think of it as a global holiday to reset the calendar cycle. In leap years, there's also 'Leap Day' which falls after June.
Your birthday would have a fixed date in the 13-month system! Use our date converter to find your new birthday. The exciting part: it will always fall on the same day of the week, every single year.
Use our Date Converter tool. Simply enter any Gregorian date and instantly see its 13-month equivalent, or vice versa.
Leap Day is inserted after the last day of June (the 6th month) and before Sol (the 7th month). Like Year Day, Leap Day doesn't belong to any month or week - it's another standalone celebration day.
The International Fixed Calendar was developed by Moses B. Cotsworth, a British accountant and statistician, around 1902. He spent decades promoting the system as a more rational way to organize time.
Yes! George Eastman, founder of Kodak, was so convinced of the calendar's benefits that Kodak used it internally from 1928 to 1989 - over 60 years.
The League of Nations seriously considered adopting it in the 1930s, but religious groups objected to the 'blank day' concept. Year Day and Leap Day would break the continuous seven-day week cycle that holds religious significance.
Use the toggle switch at the top of the calendar to switch between 'Standard' (Gregorian) and '13-Mo' (13-month) views. Both views show the corresponding date in the other system.
Our calendar offers several overlays: Moon Phases, Seasons, Sun Angle, Daylight Changes, Orbit (perihelion/aphelion), and Circadian Notes (wellness tips based on season).
Yes! Click the 'Share' button to copy a link to your current calendar view. The link preserves your selected date and view mode.
Still have questions?
Explore our interactive calendar or learn more about the project.
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