The Bench · N° 31

Annual and perpetual calendars: what happens on 1 March

An annual calendar distinguishes 30-day and 31-day months but needs help after February. A perpetual calendar also tracks the leap-year cycle, with one distant Gregorian exception.

Hen & Mills Editorial · 4 min read · 7 September 2026

The clearest difference between an annual and a perpetual calendar appears at the end of February.

An annual calendar can normally move from a 30-day month to the first of the next month without correction. It does not independently decide whether February has 28 or 29 days, so the owner corrects it once a year.

A perpetual calendar includes a programme for February and leap years. If it keeps running and is set correctly, it can advance to 1 March on its own.

What a simple date mechanism knows

A basic date display commonly assumes every month has 31 days. The date ring advances one step each night and cycles back to 1 after 31.

After April, June, September and November, the owner must move it forward from the nonexistent 31st to the first. February requires a larger correction.

This mechanism does not know the month. It only counts positions on the date ring.

What an annual calendar adds

An annual calendar tracks the month and distinguishes between 30-day and 31-day months. A programme wheel or related set of cams tells the date mechanism when to skip the 31st.

That solves four of the five routine corrections required by a simple date each year.

February remains the exception because its length changes between common and leap years. The annual calendar normally needs one manual correction at the end of that month.

What a perpetual calendar adds

A perpetual calendar tracks a four-year leap cycle as well as the month. Its mechanical memory identifies one leap year followed by three common years.

The programme directs the date to move from 28 February to 1 March in common years and to show 29 February in the leap year.

Traditional mechanisms achieve this with notched wheels, levers and cams rather than electronic calculation.

Why 1 March is the test

For most of the year, annual and perpetual calendars can behave in the same way. Both understand the alternating pattern of 30-day and 31-day months.

At the end of February, the perpetual calendar must make a further decision. It needs to know not only that February is short, but which year of the leap cycle is current.

Watching the display change to 1 March is therefore the simplest demonstration of what the additional mechanism does.

The Gregorian century rule

The modern calendar has a refinement beyond the four-year cycle. Years divisible by four are usually leap years, but century years are not unless they are also divisible by 400.

The year 2000 was a leap year. The year 2100 will not be.

Most traditional perpetual-calendar watches model the four-year rule and will treat 2100 as a leap year. If still operating then, they will need a one-time correction after February.

Secular perpetual calendars

A small number of highly complex watches include a mechanism for the century exception. These are sometimes described as secular perpetual calendars.

They add another layer of mechanical memory so the watch can suppress the leap day in years such as 2100, 2200 and 2300 while retaining it in 2400.

That is an impressive solution to a rare problem. It also adds parts, adjustment demands and cost for an event no present owner will encounter through ordinary use.

What happens if the watch stops

Neither complication remembers the date while the movement is stopped. If the watch runs down for days or months, its displays must be advanced to the current date.

Some watches offer rapid correctors. Others require the owner to move the hands through repeated 24-hour cycles or use a dedicated setting system.

A watch winder can keep the calendar moving, but it is not mandatory. Careful resetting according to the instructions is a normal part of owning the complication.

Setting restrictions matter

Calendar components begin preparing for the date change before midnight and may remain engaged for several hours. Using a quick-set corrector during that period can put stress on the mechanism.

The unsafe window varies by calibre. There is no single time range that applies to every watch.

Set the hands to a safe time specified by the manufacturer before adjusting the calendar, and never force a corrector that resists.

More displays mean more setting work

Many annual and perpetual calendars also show the day, month, moonphase and leap-year position. All of those indications must agree.

If one display is a day out, repeatedly pressing correctors without a plan can make the problem harder to diagnose. Start from a known reference and follow the maker's sequence.

A complicated calendar is easiest to live with when its setting method is clear and its pushers or crown controls are practical.

Is perpetual always better?

A perpetual calendar performs more mechanical logic and needs less routine correction while running. That makes it the more complex achievement.

An annual calendar can be thinner, simpler to service and easier to reset. One correction each year may be a reasonable exchange for those advantages.

The useful comparison includes ownership, not only the number of programmed years.

What you are paying for

Both complications replace repeated manual corrections with a mechanical programme. The annual calendar encodes the ordinary month lengths. The perpetual calendar adds February and the leap-year cycle.

On most days, their dials may tell you exactly the same thing. On 1 March, the difference becomes visible.

That is the complication in one sentence: the annual calendar needs to be told that February has ended, while the perpetual calendar is built to work it out.