The 13 month calendar is a fixed, perpetual calendar with 13 equal months of 28 days each. Every date falls on the same weekday — every year, forever. Use the interactive tool below, then convert any date, find your fixed-calendar birthday, or track the moon.
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Also called the International Fixed Calendar (IFC), this perpetual solar calendar divides the year into 13 equal months of exactly 28 days each. Thirteen times twenty-eight is 364, plus one extra Year Day at the end of December (and a Leap Day after June 28 in leap years).
Because every month is the same length and contains exactly four weeks, every date in the system lands on the same weekday every year. January 1st is always Sunday. The 15th of every month is always Sunday. Your birthday is the same weekday forever. Read the full definition →
The system replaces the uneven 28-to-31-day Gregorian months with thirteen identical 28-day blocks. Each block is exactly four weeks, every quarter is exactly 13 weeks, and the entire year is exactly 52 weeks plus one or two intercalary days.
Each month contains exactly 28 days — four clean weeks. 13 × 28 = 364, with one extra Year Day at the end.
Every month starts on Sunday and ends on Saturday. The 1st is always Sunday, the 15th is always Sunday — forever.
Sol — the new 13th month — sits between June and July, named after the sun and aligned with the summer solstice.
Year Day sits at the end of December, outside any month or week. Leap Day appears after June 28 in leap years.
The system inserts a brand-new month — Sol — between June and July, right around the summer solstice. Sol has 28 days like every other month, starts on Sunday, and ends on Saturday. It's named after the sun and gives the year its 13th equal block.
Because 13 × 28 = 364, the year needs one extra day to reach 365. That's Year Day, an intercalary day that sits between December 28 and the next January 1 — it doesn't belong to any month or weekday. In leap years, a second intercalary day (Leap Day) is added after June 28. Both exist outside the seven-day cycle, which is what keeps every weekday locked in place year after year. See the Sol month in detail →
The Gregorian calendar uses 12 months of 28, 30, or 31 days, which means weekdays shift every year and quarters are unequal (90, 91, 92, 92 days). The fixed calendar uses 13 identical 28-day months, so weekdays never shift, every quarter is exactly 13 weeks, and February's awkward leap-day rules disappear.
For payroll, billing, statistics, and personal planning, this system is dramatically simpler — every month is the same length, every paycheck covers the same number of days, and your birthday is the same weekday forever. Full comparison →
The modern system was designed in 1902 by British statistician Moses B. Cotsworth, who wanted a perpetual calendar with uniform months for accounting and statistics. In the 1920s, Kodak founder George Eastman championed it as a business standard, and Kodak used it internally from 1928 until 1989.
The League of Nations seriously considered adopting it as a worldwide standard in 1928 and again in 1937, but religious objections to Year Day breaking the unbroken seven-day week sank the proposal. Read the full history →
Translate any Gregorian date into its 13-month equivalent and perpetual weekday.
Discover what 13-month date you were born on — and the weekday it always falls on.
Current phase, next new moon, next full moon — aligned to the fixed calendar.
See the full breakdown: why 13 months instead of 12 →
Seven short guides cover every angle — definitions, comparisons, history, the Sol month, lunar overlays, and the full 2026 view.
Also called the IFC, it is a perpetual solar calendar with 13 equal months of 28 days each. That gives 364 days, plus one extra Year Day at the end of December and a Leap Day in leap years. Every date falls on the same weekday every year.
Because 13 × 28 = 364, which divides cleanly into 52 weeks of 7 days. The Gregorian system has uneven months (28, 30, or 31 days), so weekdays drift each year. The fixed calendar holds them in place — January 1st is always a Sunday, the 15th is always a Sunday, and your birthday lands on the same weekday forever.
No. It is a solar calendar — the year still tracks Earth's orbit around the sun. The 28-day month length is close to the moon's 29.5-day cycle, but the months are not synchronized to the moon.
The new month is named Sol, after the sun, and it sits between June and July (around the summer solstice). The 13 months in order are: January, February, March, April, May, June, Sol, July, August, September, October, November, December.
Year Day is the 365th day of the year. It exists outside any month or week — a one-day breather between December 28 and the next January 1. In leap years, an extra Leap Day is added after June 28 (also outside the week count). Together they keep the year aligned with the solar cycle without breaking the perpetual structure.
It was seriously considered by the League of Nations in 1928 and used internally at Kodak from 1928 to 1989. The main objection came from religious groups who opposed the Year Day concept because it breaks the unbroken seven-day week cycle.
Use the free date converter at 13cal.net/converter to translate any Gregorian date into its 13-month equivalent, including the perpetual weekday it lands on.
No country uses the system officially, but a small community of planners, businesses, and calendar reform enthusiasts use it for personal scheduling, financial planning, and lunar tracking.
More questions? See the full FAQ.
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