calendata

a calendar designer's playground

Library

Calendar def

Name

Days / week

Target year (days)

Weekday names (comma-separated)

Epoch year

…starts on weekday

The epoch fixes the weekday of day 1 of the epoch year; every other date is counted from it.

Months

Base length of each month — i.e. the length in the base year type. Year types override individual months below.

Year types

Per-month boxes override the base length for that month in this year type. Blank = inherit base. ★ marks the fallback year used when no rule matches.

Intercalation rules ↓ first match

Read top-down; the first rule a year satisfies sets its type. Gregorian: ÷400→leap, else ÷100→common, else ÷4→leap, else base.

Convert ↔ Gregorian

This calendar → Gregorian

Day

Month

Year

—

Gregorian → this calendar

Day

Month

Year

—

Show year

Months to display

Day numbers Mark intercalary days

Cycle & drift analysis

design-space & accuracy

Drift vs. true day count

from span

drift — year shorter than target, calendar falls behind snap-back — intercalary day overshoots, calendar catches up mean trend true day count (zero drift)

month-length heatmap · all year types

Calendar definition (JSON) — export / import