International Fixed Calendar

What is the International Fixed Calendar?

The International Fixed Calendar is a proposed way to organize the year into 13 months of 28 days each. Every month is exactly four weeks long. That means the 1st of every month always falls on the same weekday, the 15th always falls on the same weekday, and so on — year after year.

It is also called the Cotsworth calendar, the Eastman plan, or simply a 13-month calendar. It was designed by Moses B. Cotsworth in 1902 and later used inside Eastman Kodak from 1928 until 1989.

How 13 months of 28 days add up

The Gregorian calendar we use today has months of 28, 29, 30, or 31 days. That makes months uneven and hard to compare. The International Fixed Calendar fixes that with one simple rule:

• 13 months × 28 days = 364 days
• One extra day, called Year Day, is added after December 28 to make 365 days
• In a leap year, one more extra day is added so the year stays aligned with the seasons

Those extra days sit outside the normal week. They keep the calendar “fixed” so every month can stay exactly 28 days without drifting.

The 13th month is called Sol

Twelve month names stay the same as the calendar you already know. The extra month is inserted between June and July and is named Sol.

January · February · March · April · May · June · Sol · July · August · September · October · November · December

Sol sits in the middle of the year. That is why a date like late August on the Gregorian calendar can fall earlier in August on the International Fixed Calendar — Sol has already taken its 28 days.

Why people like 28-day months

Because every month has the same number of days and the same number of weeks, planning gets simpler:

• Every month has exactly four weeks
• Pay periods, schedules, and reports line up the same way every month
• You never have to guess which weekday a date falls on
• One month can be compared to another without adjusting for extra days

That last point is why businesses liked it. Cotsworth worked in railways and wanted monthly numbers that could be compared fairly. George Eastman adopted the same idea at Kodak for the same reason.

A short history

Moses B. Cotsworth published the plan in 1902. In the 1920s the League of Nations studied calendar reform and treated this 13-month, 28-day design as one of the leading proposals. It was never adopted by countries as an official civil calendar.

It did survive in business. Kodak used the International Fixed Calendar internally for decades. Other companies experimented with 13 equal periods for accounting even when the rest of the world stayed on the Gregorian calendar.

International Fixed Calendar vs Gregorian calendar