International Fixed Calendar or CAL13

International Fixed Calendar or CAL13 is a different calendar system than ours.

It uses 7 days a week.

With 4 weeks a month.

So each month last 28 days.

And there is 13 months a year.

Just add a white day each year as a day off to get your 365 days a year.

What does it looks like ?

Here is your monthly calendar which works for every months.

 Lun   Mar   Mer   Jeu   Ven   Sam   Dim 
1 2 3 4 5 6 7
8 9 10 11 12 13 14
15 16 17 18 19 20 21
22 23 24 25 26 27 28

There are multiple propositions for where to place the thirteen month ? and which day should start a week ?.

The most popular is to name the month Sol and to place it between June and July. The beginning of the week could be Sunday or Monday.


  • You'll know which day is the 24 of any month.
  • Each month last the same duration, no more long months and short months.
  • One extra day off !
  • No more Friday the thirteen !
  • Being synchronized with the moon.
  • Being synchronized with the average menstrual cycle.


Many people tried to ditch the Gregorian calendar.

The first person was Hugh Jones in 1745. Then Auguste Comte also proposed to adopt the Positivist Calendar in 1849. A bit closer was Moses B. Costworth in the 1900s.

Why no one use it ?

The actual system is good enough not to inflict ourselves an incredible amount of work to adopt it. There is a lot of inertia to fight in order to change as usual. It's the same old story as the US adoption of the metrics system.

The Kodak Company relied on this calendar between 1928 and 1989… it probably was a tremendous hassle to work with the two systems in parallel.

On top of the difficulty to use a new system, it would be even harder for historians to play with a new calendar to synchronize old dates to new ones.

What if instead of using a wrong starting reference (no Jesus wasn't born on the first of January 0 (nor the 25 of December -1)) we used a new reference ? The Holocene Calendar chooses to start in -10 000 with the start of the human civilisation.