Watches that tell the day, date and month are among the most eternally popular and practical choices money can buy. But the differences between them may not be apparent at first sight. Allow us to explain what makes perpetual calendars, annual calendars and triple – or “complete” calendars – so prized.

The calendar watch’s mysterious ability to tell the correct date in spite of the vagaries of our calendar, vagaries that still have the best of us muttering a rhyme come the 30th of each month, exerts an allure stemming from the dawn of timekeeping itself. The most complicated calendar of all, the perpetual calendar, takes leap years into account thanks to a wheel that completes a single rotation every four years. That’s practically tectonic.

As with most matters when it comes to timekeeping, it all boils down to the heavens. The sun, the phases of the moon and the observable stars were the only timekeepers that mattered for centuries, so it was inevitable that watches from the mid-16th century began to feature astronomical complications, including the day, date and phases of the moon. Their modern descendants are no different, and creating a perpetual calendar remains a yardstick by which the best watch brands are judged.  

The most sophisticated of calendar watches, the perpetual calendar is programmed to understand the different lengths of months and even leap years, only needing adjustment once a century. Due to the quirks of the Gregorian calendar – where one leap year is deleted every 100 years – your great-grandsons may still have to manually adjust your bequeathed wristwatch come 28 February in 2100.

(A note on naming conventions: perpetual calendars are sometimes referred to as “QPs” after the French abbreviation for the complication, quantième perpétuel. And it has nothing to do with Rolex’s use of the term “perpetual”, which it uses to describe its automatic-winding Oyster watches.)

It is believed to have been invented in England, London being the capital of 18th-century chronometry, after all. Perpetual calendars had already featured in clocks for some decades, but in 1762, Mr Thomas Mudge, the inventor of the still-ubiquitous “lever escapement” mechanism, made what is thought to be the first portable perpetual-calendar watch. This sold at Sotheby’s in 2016 for the trifling sum of £62,500.

But it was the ateliers of the Vallée de Joux, the Alpine cradle of complicated Swiss watchmaking, where this complication truly came to life in the 19th century. By the 1930s, Audemars Piguet and next-village neighbour Jaeger-LeCoultre had managed to shrink things into wristwatch size, toying with cushion shapes and art-deco streamlining, before 1955 saw the very first series of perpetual calendars to feature the now-essential leap year indication.

Come the 1980s, after the arrival of quartz technology had wrought havoc in the Swiss watch industry, IWC SCHAFFHAUSEN’s resident genius Mr Kurt Klaus singlehandedly revived interest in horology’s most romantic complication with his own revolutionary take on the perpetual calendar. Programmed for 500 years, set via the crown only rather than pushers (most perpetual calendars require separate adjustment for the days, months and years via discreetly hidden push-buttons set flush into the side of the case), and boasting a four-digit year display – a first in the watch industry – this perpetual calendar was so perfectly conceived that it’s still a crucial player in IWC’s lineup.

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Most of the major advancements in fine watchmaking were made a long time ago. The stopwatch chronograph? Minted by Mr Louis Moinet in 1816. The merry-go-round tourbillon? Mr Abraham-Louis Breguet, 1801. The perpetual calendar? See above.

There is one complication that doesn’t fit that mould, however. It might sound like it’s been around forever, but the annual calendar was actually introduced in 1996. As the name implies, it needs adjusting just once a year.

Instead of taking into account February’s 28 days as well as leap years, an annual calendar's date display takes into account the four months lasting 30 days rather than 31, so the only time you need adjust the date is on the 28 or 29 February, otherwise the mechanism “assumes” that February is always 30 days long.

Nowadays, annual calendars are found at A. Lange & Söhne, Cartier, Zenith, Vacheron Constantin and IWC SCHAFFHAUSEN to name just a few. None of these watches would have existed, though, without the company that came up with the annual calendar in the first place – Patek Philippe.

At launch, top brass from the Genevan grande dame said, “The Ref 5035 will appeal to value-conscious watch enthusiasts,” with uncommon transparency. Simply put, they wanted to make more watches, more quickly, which meant finding a complicated function that was easier to make than the perpetual. The trick was that the annual calendar mechanism itself was modular, and could therefore be assembled onto the brand’s pre-assembled base calibres by less skilled watchmakers. (The irony being the new movements had more parts than the “integrated” movements inside their perpetual cousins.)

Unlike the annual or the perpetual calendar, the complete calendar – sometimes also referred to as a “triple” calendar, for its display of day, date and month – does not take into account the variable lengths of the months and needs to be corrected five times a year, at the end of those months shorter than 31 days.

However, it’s still a significant step up from a normal date or day/date window, chiefly for aesthetic reasons. There’s plenty of added symmetry as well as design opportunities lent by two day and month windows, with either a small-handed date sub dial or centre-axial long-handed “pointer” date indication running the circumference of the dial. Often a moon phase will be thrown in for good measure – usually inside the date sub dial at six o’clock – bringing extra sophistication to an otherwise highly affordable watch.

Highly popular from the 1940s onwards, today revival models from the likes of Jaeger-LeCoultre and Vacheron Constantin have found favour with a new audience. Indeed, the vast majority of complete calendars cleave closely to this aesthetic.

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