Perpetual Calendar Checker – Leap Year & Month-End Guide
Pick a date. See month length, leap-year logic, and whether a simple date, annual, or perpetual calendar should need your hands on the crown.
What each calendar type does
Simple dateOften assumes 31-day months. You may need to correct after shorter months and February.
Annual calendarKnows 30- vs 31-day months. Usually still needs one correction after February (non-leap and leap handling varies by design).
Perpetual calendarTracks month lengths and leap years so normal civil dates advance without monthly babysitting.
Gregorian leap rule: year ÷ 4 yes, unless ÷ 100 then no — unless also ÷ 400 then yes. 2000 leap · 1900 not · 2100 not.
Educational civil-calendar logic for collectors. Always follow your specific movement’s setting manual (danger zone still applies).
Month-end cheat sheet
- 31-day months: Jan, Mar, May, Jul, Aug, Oct, Dec
- 30-day months: Apr, Jun, Sep, Nov
- February: 28 days, or 29 in a leap year
- Century years: leap only if divisible by 400
Frequently Asked Questions
Does a perpetual need adjusting? Not for ordinary month-ends and leap Februaries the way a simple date does.
Annual vs perpetual? Annual still usually needs a February correction; perpetual handles leap years too.
Leap year rule? ÷4 yes; ÷100 no; ÷400 yes.