Earth's tropical year as reference.
Seasonal Drift Over Time
Long-Run Accuracy
Avg year length—
Target year365.2422 d
Drift per year—
Drift after N yrs—
1 day off in—
Worldbuilders constantly invent time systems for their settings — a lunar year of 354 days, a decimal calendar of 360, a 10-day week, a leap rule nobody has thought through. But the interesting part of that work is the consequences: does the year divide evenly into weeks? How fast does the calendar drift away from the seasons? How long is the leap cycle and how accurate is it? Those are exactly the questions that make a fictional calendar feel real, and there was no single tool that let you design the system and instantly see its consequences.
Calendar Forge is a calendar-system designer for fictional worlds. You specify the raw ingredients — year length, number and length of months, the week, and the leap rule — and the tool generates the whole era: a browsable year grid, live statistics, a long-run drift chart against a fixed seasonal anchor, and a shareable code that encodes everything.
The calendar engine is a small pure-JavaScript model. The year length is split into months (evenly, alternating long/short, or a custom list). The leap rule is evaluated year by year using the standard Gregorian-style "divisible by A, except B, unless C" test (or a simple every-N-years rule). The week grid is built by walking day-by-day, so any year length and week length work — including combinations where the year does not divide evenly, which the tool flags so you know your New Year will slowly slip through the weekdays.
Drift is computed in days: each simulated year advances a running total against a target (Earth's 365.2422-day tropical year by default), and the chart plots that accumulated offset. Leap-cycle accuracy is measured the same way your real calendar was engineered — average length over one full cycle versus the target, reported as days-of-drift per century.
Sharing uses a compact, self-describing encoding (base64url of the parameter set) embedded in the URL hash, so the link is the entire save file. Everything runs in the browser — nothing is uploaded.