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Fantasy Calendar System Generator

A calendar is not a list of month names. It is a piece of arithmetic that has to balance, and the reason real ones are strange — a February, a leap year, a rule that skips a leap year every century — is that the year does not divide into days evenly and somebody had to decide where the remainder goes. This builds the arithmetic first: months whose lengths vary and still add up to the year exactly, a leap rule chosen to be the most accurate one of its length, and moons whose cycles are worked out night by night rather than asserted.

What this generator does

Divides the year you ask for into months of differing lengths that still total it exactly, works out the most accurate leap rule available for a world whose true year is a whole number of days plus a fraction, and gives each moon a cycle. Then it takes dates apart and puts them back together to confirm the calendar closes.

How to use this tool

  1. Choose how many months a year has, how many days, and how many moons hang over it.
  2. Build the calendar, and read the months, the week, the leap rule and the moons.
  3. Look at the drift figure: it says how far the calendar will have slipped after a thousand years.
  4. Copy the link to keep this calendar, or share it with everyone at your table.

Understanding the controls

Months in a year
Between 6 and 16. Lengths are varied by moving days between months, so the total cannot drift while the shape does.
Days in a year
The whole days counted before the leap rule. The world’s true year is this plus a fraction, which is where the leap rule comes from.
Moons
One to three. With two or three the night they are all full together is often centuries apart, and that night is found by walking the days rather than by assuming it.
Repeat word
The word this calendar was built from. Anyone opening the same link gets the same calendar.

Common use cases

  • Dating a campaign consistently across a whole season of play
  • Giving a world a festival calendar that will not fall apart when a player does the arithmetic
  • Writing a novel where time passes over centuries and the seasons still land
  • Teaching what a leap year is actually for, on a year nobody already knows the answer to
  • Handing every player the same calendar from one link

How this generator works

The leap rule is found by trying every rule of the form ‘add so many days every so many years’ up to a cycle of four hundred, and keeping the most accurate — so the rule is provably the best one a world could state, and the drift figure is a subtraction rather than a claim. Worth knowing: the Gregorian rule of 97 days in 400 years is not the most accurate rule of that length. 31 days in 128 years is closer. Real calendars are compromises with arithmetic people can do in their heads.

Randomness and fairness

The calendar is built from a repeat word drawn from your browser’s secure generator, unless you type one of your own. That makes a calendar reproducible from a link, which is the point of it, and it means the sequence behind it is deterministic and not cryptographic.

For how randomness is produced across the whole site, see how Generate Random works.

Limitations and good to know

  • Months, days of the week and moons are invented. No real calendar, culture or astronomy is being described.
  • The moons’ cycles are whole numbers of days, which is a stylisation — a real lunar month is not.
  • The leap day always falls in the same month, so the calendar has no equivalent of a leap week or an intercalary month.
  • The drift figure measures the calendar against its own stated true year, not against any real season.

Privacy and your data

The calendar is built on your device and nothing about it is transmitted or stored. The repeat word is a short label this page made or you typed, and it is the only thing that appears in the address; it is never measured.

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