What the 13-Month Calendar Is (and Is Not)
A clear, rational definition of the fixed 13-month calendar system
Key Takeaways
- The 13-month calendar has 13 months of exactly 28 days each (364 days total)
- Every date falls on the same weekday, every year, forever
- It is NOT lunar, religious, or astrological - it is a solar arithmetic calendar
- Year Day (and Leap Day in leap years) exist outside the weekly cycle
What Is a 13-Month Calendar?
A 13 month calendar is a calendar reform proposal that divides the year into 13 months of exactly 28 days each. The most well-known version is the International Fixed Calendar (also called the Cotsworth Plan), developed by British statistician Moses B. Cotsworth in 1902.
The core idea is simple: 13 months multiplied by 28 days equals 364 days. Since a solar year has approximately 365.25 days, the system adds one or two extra days that exist outside the normal week structure. These are called "Year Day" (added every year after December 28th) and "Leap Day" (added in leap years after June 28th).
The 13th month is called "Sol" and is inserted between June and July. Every month starts on a Sunday and ends on a Saturday. The 1st of every month is always Sunday, the 15th is always Saturday, and so on. This consistency is the calendar's defining feature.
What the 13-Month Calendar Is NOT
Many people confuse the fixed 13 month calendar with other calendar systems. Here is what it is not:
Not a Lunar Calendar
The 13-month calendar is entirely solar-based. While the 28-day month happens to be close to the lunar cycle (~29.53 days), this is coincidental. The calendar does not track moon phases. For more on this distinction, see our detailed explanation.
Not Religious or Mystical
This is a practical, arithmetic system designed for business and planning efficiency. It has no spiritual, astrological, or religious basis. The number 13 is simply a mathematical consequence of dividing 364 by 28.
Not the Same as Historical 13-Month Systems
Various cultures have used calendars with 13 periods, often based on lunar months. The International Fixed Calendar is distinct - it is a modern proposal specifically designed for administrative efficiency, not cultural or astronomical tracking.
Why People Like the 13-Month System
The primary appeal of the 13-month calendar is predictability. In the Gregorian calendar, dates shift weekdays each year, months have irregular lengths, and planning across years requires constant recalculation. The fixed calendar eliminates all of this.
Key benefits include:
- Fixed weekdays: Your birthday falls on the same day of the week every year
- Simplified accounting: Every month has exactly 4 weeks, making payroll, rent, and billing uniform
- Cleaner quarters: 13 weeks per quarter (one extra month rotates through quarters)
- No calendar reprints: The same calendar works every year
- Easy mental math: The 14th is always Saturday, the 21st is always Sunday
George Eastman, founder of Kodak, was so convinced of these benefits that Kodak used the 13-month calendar internally from 1928 to 1989.
Handling the Extra Days
Since 13 x 28 = 364 and a year has ~365.25 days, the calendar needs special handling for the extra days:
Year Day (Every Year)
December 28th is followed by Year Day, which is not part of any week or month. It falls between December 28th (Saturday) and the next year's January 1st (Sunday). Think of it as a worldwide holiday that exists outside normal time.
Leap Day (Leap Years Only)
In leap years, an additional day is inserted after June 28th (the last day of the 6th month) and before Sol 1st. Like Year Day, Leap Day does not belong to any week, preserving the Sunday-to-Saturday structure of every month.
This "blank day" concept was the main reason the calendar was not adopted globally. Religious groups objected because it would break the continuous seven-day week cycle that has religious significance in Judaism, Christianity, and Islam.
Compared to the Gregorian Calendar
The differences between the 13-month and Gregorian calendars are substantial. The Gregorian calendar has months ranging from 28 to 31 days, weekdays that shift annually, and quarters of unequal length. The 13-month calendar solves all of these irregularities.
Use our date converter to see how any Gregorian date maps to the 13-month system.
Frequently Asked Questions
Is the 13-month calendar used anywhere today?
No country officially uses it, but individuals and some organizations use it for personal planning. It remains a topic of interest among calendar reform enthusiasts.
What is the 13th month called?
The 13th month is called "Sol" (from the Latin word for sun). It falls between June and July in the calendar.
Would my birthday change?
Your birthday would have a new date in the 13-month system, but it would always fall on the same weekday. Use our converter to find your new birthday.