13maandenjaar

The 13maandenjaar, or "13-month year," is a concept that proposes a calendar reform where the standard year is divided into 13 months, each consisting of 28 days. This structure would result in a year of 364 days (13 months * 28 days/month). The remaining day (or two in a leap year) would be an extra day, outside of any month, to align with the solar year.

Proponents of the 13maandenjaar argue that its regular structure simplifies scheduling and calculations. With every month having exactly four weeks, the day of the week for any given date would remain consistent from year to year. For example, if January 1st is a Monday in one year, it would also be a Monday in the next. This predictability is seen as a significant advantage for businesses, governments, and individuals.

The concept has roots in early 20th-century proposals, notably by Moses B. Cotsworth. While it has gained some traction and discussion, the 13maandenjaar has never been widely adopted. Significant logistical and cultural challenges, including the need for global consensus and the disruption of existing systems, have prevented its implementation. The traditional Gregorian calendar, with its uneven month lengths, remains the dominant global standard.