A little background on the history of calendars is vital to understanding the importance and value of one of Patek Philippe’s most renowned complications – the perpetual calendar, the subject of a recent seminar in Singapore organised by the Geneva watchmaker.

Calendars – the world’s earliest horological devices – were the result of our ancestors’ attempt to divide time, a task that took centuries to complete. Early units of time were organized by observing astronomical cycles, which helped mankind regulate his existence, aiding agriculture, hunting and the observing of religious holidays.

A calendar begins with three natural rhythms that form the basis of what perpetual calendars accomplish, starting with the speed at which the Earth rotates on its axis, which forms the day. A larger rhythm is the moon’s orbit around the Earth that gives us the month. And finally the Earth’s orbit around the Sun, a route that takes approximately 365 days, or one calendar year.

Relying on complex formulas, the intricate gears and springs inside a perpetual calendar flawlessly amalgamate the three independent cosmic rhythms. Horology is thus synonymous with astronomy, charting the relationship between the Sun, Moon, and Earth, cyclical in pattern and perpetual in nature.

Patek Philippe perpetual calendars see the lunar calendar depicted in the moon phases, and the solar Gregorian calendar indicated in the date windows on the dial.

Special gearing is required to account for irregular occurrences, such as the varying lengths of months – the pesky 28-day month anomaly instead of 30 or 31 – and the leap year once in every four years, making it a considerable step up from annual calendars, which cannot handle anything outside of 30- and 31-day months.

It also means that as long as the watch is kept wound and running, a perpetual calendar only need to be adjusted once a century.

ONE: The first perpetual calendar wristwatch was a Patek Philippe

Patek Philippe did not invent the perpetual calendar, English watchmaker Thomas Mudge did in the 18^th century. In fact, Sotheby’s sold an example of a Mudge perpetual calendar pocket watch dating to 1762 – perhaps the oldest one ever – just last year for £62,500.

It took Swiss watchmakers a century or so to catch up, with one of the earliest perpetual calendars by Patek Philippe being a pocket watch manufactured in 1864 that’s now in the company museum. Fitted with an enamel dial, the pocket watch features an instantaneous date, retrograde date, day, month indications in Spanish and moon phase.

Patek Philippe does, however, lay claim to the first ever wristwatch with a perpetual calendar. It was produced in 1925, and came about due to a good deal of chance. Patek Philippe made the movement in 1898, but for a women’s pendant watch. However, the pendant watch remained unsold for lack of interest so in 1925 Patek Philippe decided to put the compact movement into a wristwatch case. At only 34.4mm in diameter, the watch was finally sold in October 1927 to Thomas Emery, an American who also owned several other high-end Patek Philippe timepieces.

The first perpetual calendar wristwatch, circa 1925

The Emery perpetual calendar, however, was a one-off. Patek Philippe only began serial production of perpetual calendar wristwatches with the introduction of hand-wound ref. 1526 in 1941. It took another 21 years for the first automatic perpetual calendar to hit the market, with the ref. 3448 that was unveiled in 1962.

One of the earliest examples of the ref. 1526, produced in 1942

In more recent times Patek Philippe combined the perpetual calendar with other complications – including the landmark perpetual calendar chronograph that is synonymous with the brand – resulting in some of the most sought after watches in the world. In fact, the most expensive watch ever sold at auction is a stainless steel ref. 1518 chronograph with perpetual calendar.

TWO: Calendar displays, three ways

Patek Philippe perpetual calendars feature complex dials with various types of displays, namely aperture-based and sub-dials. Aperture displays include the triple in-line and twin in-line, which get their name from the calendar windows arranging linearly on the dial.

A twin in-line display on a ref. 3448, circa 1962

A 1928 perpetual calendar pocket watch with aperture display, powered by a movement built on a Victorin Piguet ebauche.

Arguably the quintessential perpetual calendar aesthetic, the sub-dial based displays typically have three smaller dials for each of the calendar functions. This style began with the ref. 3940 of 1985 powered by the cal. 240 Q, and has now evolved into over a dozen different models.

The ref. 5327J, the latest iteration of the classic perpetual calendar with sub-dials

And the third type of display often found on Patek Philippe perpetual calendars combines apertures with a retrograde date display. The first serially produced watch to feature such a display was the ref. 5050 introduced in 1993, though Patek Philippe did create a one of a kind example in 1937.

The modern day ref. 5160/500G with a retrograde date and hand-engraved case

THREE: Three distinct mechanisms

Just as the displays differ, the manner in which the indicators change are similarly diverse. Perpetual calendars are executed with various types of jumping indicators. The simplest is the dragging, where the indicator changes slowly over a period of time. A dragging date display, for instance, will take several minutes to change at midnight.

Then there is the semi-instantaneous, which has both dragging displays for some parts of the calendars and instantaneous jumps for others. This is the most common sort of mechanism, found in most Patek Philippe perpetual calendars, including the recent ref. 5230G.

The ref. 5320G, just launched at Baselworld 2017

The most impressive of all is the instantaneous perpetual calendar, which Patek Philippe introduced in 2008 with the ref. 5207P. That was followed by the ref. 5208P (a favourite of Thierry Stern’s and often on his wrist) in 2011, which featured the addition of a mono pusher chronograph.

Rather than creeping forwards slowly as a dragging display does, the calendar hands jump instantaneously at midnight. In an instantaneous calendar, all of the calendar functions jump simultaneously at midnight (albeit with an allowance of two minutes on either side of 12), requiring complex mechanics that store power throughout the day before releasing it to trigger the calendar mechanism.

Sarah Ho is a Singaporean studying at University College London (UCL) who’s also a budding writer and passionate theatregoer.