13month

13month refers to a calendar proposal in which the year is divided into thirteen months of 28 days each. Each month would contain four weeks, totaling 364 days. To align with the solar year, additional days are introduced outside the regular months: a Year Day at the end of the year, and in leap years an extra Leap Day. These outside days are not assigned to any week, while the thirteen 28-day months provide a uniform, repeatable structure for scheduling and timekeeping.

The most prominent variant is the International Fixed Calendar, developed in the early 20th century by Moses B. Cotsworth. It envisions 13 months of 28 days and uses the Year Day outside the monthly cycle; in leap years an additional day outside the weeks would be added. The idea gained occasional attention from reform advocates and some businesses during the 20th century but never gained widespread adoption or formal civil implementation.

Reception to 13month systems has centered on practical challenges. Critics point to disruptions in religious and cultural holidays, seasonal and fiscal cycles, and the substantial changes required for education, public administration, and computer systems. Proponents argue for simplified arithmetic, consistent month lengths, and easier planning across years. In contemporary use, 13month calendars remain mostly of historical interest, used in discussions of calendar reform, in speculative writing, and by hobbyists or researchers exploring alternative timekeeping. The Gregorian calendar remains the global standard, and no 13-month proposal has achieved broad official adoption.