30 days this month
Explore the idea
A year that fits on one page
Understand the International Fixed Calendar, compare dates and see where history, astronomy and our project choices differ.
28 days every month
13 equal months. 28 days each.Every month starts on Sunday. Every month ends on Saturday.The calendar that almost replaced the one you use today.
The structure
The whole idea in 30 seconds
Thirteen equal months of 28 days make 364 regular days. Year Day, and Leap Day when needed, sit outside the months and the civil week.
13 equal months
Because 28 divides perfectly into 4 weeks. No other month length does that. 13 × 28 = 364, which is as close to 365 as you can get with perfectly equal months.
A month called Solaris
It sits between June and July. Named after the sun. Everything else keeps its name — January is still January, December is still December. There's just one more month in between.
Year Day
13 × 28 = 364, so there's one day left over. That's Year Day — it comes after December 28 and before next January 1. It's not a Monday or a Tuesday. It's just a day off. A universal holiday.
Leap Day
Same rule as now — every 4 years, you get an extra day immediately after Year Day. It also sits outside the weekly cycle. Two holidays instead of one.
The week never breaks
Every month starts on Sunday and ends on Saturday. The 1st is always a Sunday. The 14th is always a Saturday. Year after year after year. You could throw away your calendar and still know what day it is.
This is what every month looks like
All 13 of them. Exactly the same. No exceptions.
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4 weeks. Always starts Sunday. Always ends Saturday.
Full Calendar
2026 at a glance
All 13 months laid out — each one identical, each one perfectly predictable.
January
1SunMonTueWedThuFriSat
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February
2SunMonTueWedThuFriSat
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March
3SunMonTueWedThuFriSat
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April
4SunMonTueWedThuFriSat
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May
5SunMonTueWedThuFriSat
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June
6SunMonTueWedThuFriSat
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Solaris
7SunMonTueWedThuFriSat
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July
8SunMonTueWedThuFriSat
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August
9SunMonTueWedThuFriSat
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September
10SunMonTueWedThuFriSat
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October
11SunMonTueWedThuFriSat
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November
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December
13SunMonTueWedThuFriSat
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Special days
2026 is not a leap year- Regular days
- 364
- Year Day
- +1
- Leap Day
- —
- Total
- 365 days
Interactive converter
Try any date
Both directions use the same calculation engine as the main calendars.
IFC date
Sunday ·September 152026Month 10 of 13 · Week 3 of 4Shared calculation engine
In regular IFC dates, the same month and day always return to the same civil weekday.
This isn't new
People have been trying to fix the calendar for over a century. It almost happened.
An accountant has an idea
Moses Cotsworth, a British railway statistician, publishes 'The Rational Almanac.' His argument: months should be equal. He designs a 13-month calendar to prove it.
The League of Nations looks into it
Calendar reform becomes a real international conversation. A formal committee is set up to evaluate proposals, and the 13-month calendar is one of the front-runners.
Kodak starts using it
George Eastman loves the idea so much he switches Kodak's entire internal operations to 13 months. Payroll, accounting, everything. It works.
International reform loses momentum
Religious objections to days outside the weekly cycle were a central obstacle.
Kodak gives up
After 61 years, Kodak drops the calendar. Not because it didn't work — it did. But being the only company using a different calendar got old.
Still a good idea
Software runs everything now. Switching calendars is a database migration, not a civilization-level crisis. The hard part was never the math.
Yeah, but...
What about the leftover day?
It's called Year Day. It comes after December 28 and before the next January 1. It doesn't belong to any week — no Monday, no Tuesday, just a day off. In leap years, Leap Day comes immediately after Year Day.
Do holidays move?
Some dates would shift, but here's the thing — every date would always be the same day of the week, forever. Christmas on a Wednesday? It's on a Wednesday every year. That's more predictable, not less.
Doesn't this break the Sabbath?
This is the objection that killed it in 1937. Year Day sits outside the 7-day week, which some see as breaking an unbroken chain. Others say it's just a pause. It depends on who you ask.
Has anyone actually tried this?
Kodak ran on it for 61 years. It wasn't a pilot program — it was how they did business. Payroll, planning, accounting. They only stopped because the rest of the world wouldn't switch.
Why 13? Why not 10 or something?
Because 28 divides perfectly into 4 weeks. No other month length does that. 13 × 28 = 364, which is as close to 365 as you can get with perfectly equal months.
Could we actually do this?
The math has never been the problem. In 1937 it meant reprinting every calendar and contract on earth. Today it's a software update. The barrier is cultural, not technical.
A continuous week, for those who need it
What about the Sabbath?
In 1937, the League of Nations — not the UN, which was founded in 1945 — removed calendar reform from its agenda after strong religious objections. A central concern was that Year Day and Leap Day, placed outside the civil week, would interrupt the uninterrupted seven-day count.
Two cycles can coexist
The civil calendar can keep its fixed months and weekdays while an optional religious cycle continues counting every sunrise in groups of seven. It would work as a parallel calendar used mainly by observing communities, comparable to lunar or lunisolar calendars such as the Jewish calendar rather than a general civil calendar.
Benefit: fixed civil dates remain predictable for everyone.One continuous civil week
Give Year Day and Leap Day ordinary weekday positions. Civil and religious time then share one uninterrupted seven-day cycle.
Trade-off: after each special day, dates change weekday and later months no longer always begin on Sunday.Civil calendar
Every month still begins on Sunday and ends on Saturday.
Result: every civil month preserves the same Sunday-to-Saturday pattern.Continuous observance
The Sabbath remains every seventh day, even when its observance no longer matches the civil label Saturday or Sunday after a special day.
Trade-off: religious observance and civil weekday labels diverge after a special day.Civil labels
Saturday 28Year DaySunday 1
Continuous cycle
7th day1st day2nd day
Historical sources: JTA, 1 Oct. 1937 · United Nations, 1945
Planning without forcing the months
How do fiscal and academic periods fit?
Thirteen equal months do not divide cleanly into halves, thirds or quarters by month count. The stable unit is the week: the 364 regular days make exactly 52 weeks.
364 regular days52 weeks4 quarters of 13 weeks
Four equal fiscal quarters
Each quarter would have 13 weeks, or 91 regular days. Closing may fall inside a fixed month, but every report would cover an equal period.
4 × 13 weeks = 52 weeksThree- or four-month reports
The 13 months still serve monthly detail. For consolidation, a company would publish the week interval and corresponding civil dates instead of creating unequal groups of months.
Months for detail; weeks for consolidationTwo-month periods, terms and semesters
First define the number of teaching weeks. In a 40-week example, that means two 20-week semesters, four 10-week periods or three terms of 13, 14 and 13 weeks; the rest of the year remains for vacations, exams and transitions.
40 = 2 × 20 = 4 × 10 = 13 + 14 + 13Vacations and academic calendars
The school year does not need to occupy the entire civil year. Classes, assessments, vacations and recesses can be scheduled by week and then adapted to local requirements.
Local law still prevailsOne year, two planning layers
See how the 52 weeks can be grouped
Switch between fiscal and school planning. Each mini month has four weeks; colors show the active period without changing the calendar months.
Fiscal
School/academic
13 + 14 + 13 = 40 teaching weeks1st: months 2–4 + week 1 of month 5. 2nd: the rest of month 5 + months 6 and 8 + three weeks of month 9. 3rd: the rest of month 9 + months 10–12.
1 2 3 4
1 2 3 4
1 2 3 4 🍁
1 2 3 4
1 2 3 4
1 2 3 4
1 ❄️2 3 4
1 2 3 4
1 2 3 4
1 2 🍀3 4
1 2 3 4
1 2 3 4
1 2 3 ☀️4
Term 1 Term 2 Term 3 Vacation · 12 weeks in months 1, 7 and 13
Hemisphere used for seasonal emojis
North
South
🍁Autumn · March equinox❄️Winter · June solstice🍀Spring · September equinox☀️Summer · December solstice
Official examples of fiscal years and school periods organized by weeks or teaching blocks: IRS, 52–53-week tax year · Amapá, 40 semanas letivas · NOAA, estações
The passage between two years
What happens to the days outside the months?
Year Day — and Leap Day when present — sits after Saturday 28 of the thirteenth month and before Sunday 1 of the new year. Its social and labor status therefore needs to be defined explicitly.
Saturday 28Year DayLeap Day leap years onlySunday 1
Celebration and shared transition
Year Day can be a public holiday for community celebrations, family, culture and the close of the civil year.
Rest with essential services
Paid rest is the clearest default. Health, safety, transport and other essential services would operate with shifts and compensation.
Leap Day with the same protection
In leap years, the additional day can follow the same rule, with a lighter program of volunteering, culture or environmental activities.
Regular work would remain possible
A country or employer could treat them as ordinary workdays, but would need rules for pay, deadlines, weekly rest and systems because they have no weekday label.
Predictability without erasing history
The observance date and the day off need not be the same
A commemoration can stay on its historical, cultural or religious date while the work holiday is observed in a predictable window. Any transfer would need explicit law and would not suit every occasion.
Original observance date preserved
Calendars, ceremonies and education keep the original date and meaning. The historical or religious reference is not erased.
Day off observed predictably
Where law allows, only the paid day off may move to a Monday or Friday known in advance in every month.
Example in any fixed month
Two predictable windows
Because every month starts on Sunday and ends on Saturday, the first Monday is always day 2 and the last Friday is always day 27.SunMonTueWedThuFriSat12345678910111213141516171819202122232425262728
First Monday · day 2Last Friday · day 27
One possible public rule
Preserve first; move only when appropriate
- 1Record and celebrate the holiday on its original date.
- 2Define by law which holidays allow only the day off to move.
- 3Choose the first Monday or last Friday, with exceptions for religious, astronomical or date-essential observances.
From opinion to adoption
How would you solve the obstacles?
The Sabbath and the uninterrupted week were a central objection. Adoption would also require answers to other practical and cultural changes:
Share a solution
How would you address the Sabbath and the other obstacles? Your contribution is optional.
Your view of the idea
What do you think of this idea?
Would you switch to a 13-month calendar?
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