History of the 13 Month Calendar
From Moses Cotsworth and Kodak to the League of Nations vote
Origins of the 13 month calendar
The modern 13 month calendar was designed in 1902 by British-born accountant and statistician Moses Bruine Cotsworth. Working with railway statistics in Yorkshire, Cotsworth grew frustrated with the uneven Gregorian months (28, 30, or 31 days), which made month-over-month comparisons mathematically dishonest. His solution: a perpetual calendar of 13 equal months of 28 days each, plus one extra Year Day to round out the solar year.
Cotsworth published The Rational Almanac in 1902, laying out the full 13 month calendar in detail. He proposed naming the new 13th month "Sol" and inserting it between June and July, near the summer solstice. The proposal got little traction in Britain, but it would soon find a powerful champion across the Atlantic.
George Eastman and Kodak adopt the 13 month calendar
In the 1920s, George Eastman, founder of the Eastman Kodak Company, became the most influential business advocate for the 13 month calendar. Eastman saw immediately how 13 equal months would simplify payroll, billing, statistics, and production scheduling. He funded research, wrote articles, and personally lobbied governments and the League of Nations to adopt the system.
Kodak put its money where its mouth was: the company switched its internal accounting and operations to the 13 month calendar in 1928. Kodak ran on 13 equal 28-day months for over 60 years — every internal report, every payroll cycle, every production run — until finally returning to the Gregorian calendar in 1989.
The League of Nations vote on the 13 month calendar
The 13 month calendar reached its closest brush with global adoption at the League of Nations in the late 1920s and 1930s. The League's Special Committee of Enquiry into the Reform of the Calendar studied dozens of proposals; the 13 month calendar (the International Fixed Calendar) emerged as the leading candidate.
In 1928, the League conducted formal consultations with member nations. A second major review followed in 1937. Both times, the 13 month calendar narrowly failed to pass — not on mathematical grounds but on religious ones. Jewish, Christian, and Islamic authorities objected because Year Day breaks the unbroken seven-day week cycle that has religious significance. The proposal was shelved, then quietly abandoned as World War II consumed international attention.
After Kodak: the 13 month calendar today
When Kodak dropped the 13 month calendar in 1989, it lost its only major institutional user. But the system never disappeared. It survives in three places: fiscal calendars (many retailers still run on 4-4-5 or 13-period accounting that is essentially the 13 month calendar with different labels), calendar reform societies, and personal planning communities who use it for perpetual scheduling and lunar tracking.
More than a century after Cotsworth wrote The Rational Almanac, the 13 month calendar remains the cleanest, most mathematically honest perpetual calendar ever proposed.