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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