IN THE Gregorian calendar every fourth year is a leap year unless this fourth year happens to fall at the end of a century. In which case it must be exactly divisible (no decimals in the sum) by four, 100 and 400. For instance, 2096 is a leap year but 2100 is not — and this is why perpetual calendar watches need corrections on March 1, 2100. Put another way, the Gregorian leap cycle of 400 years counts 97 leap days — that will be February 29 — and 71 years with an additional 53rd week in them (an average year has exactly 52.1775 weeks). These mechanisms ensure the Gregorian calendar remains accurate to within a day in the course of more than 3,000 years.

Patek Philippe, in creating its new ref. 5212A-001 Calatrava Weekly Calendar, chose to account for this so-called leap week which comes every five or six years (every 5.63 years during a 400-year cycle). This can readily be seen on the watch’s week track, which lists the odd-number years in numerals and identifies the even-number ones with dots. At the top of this scale is “53.” A month track surrounding the week track marks which weeks belong in which month.

This leap week system was adopted in ISO 8601 (even if the international standardization group shies away from calling it as such). ISO 8601 also sets Monday as the start of the week rather than Sunday, and identifies the first week of the year as the one in which January 4 falls, which may differ in some years from the Gregorian calendar. The Calatrava Weekly Calendar adheres to this standard.

Now why are these features notable? It’s simply because Patek Philippe has long been regarded for its mastery of calendar complications — its perpetual calendar watches are the brand’s specialties, and it even has an extremely complex secular perpetual calendar that is preprogrammed into the 28th century. In 1996 it launched the patented Annual Calendar, a complication which “knows” which months in a year have 31 days and which have 30. So, in choosing to adopt ISO 8601, Patek Philippe has again likely set the template for the weekly calendar complication.

Allowing for this new complication which, by the way, also displays the day and date, is a totally new semi-integrated mechanism comprised of 92 parts. Patek Philippe explains a seven-point star on the hour wheel at the center of the movement controls the day display. A second seven-point star drives the 53-tooth week wheel via a lever. This week wheel has one tip longer than the others — for Sunday — so the calendar would know when to move the week hand to the next week (which starts on a Monday, as previously noted).

The day and week displays can be corrected via a pair of pushers at 8 o’clock and 10 o’clock (the date, as usual, is adjusted via the crown). Patek Philippe says a mechanism allows corrections any time without the acts damaging the movement.

Speaking of which, this movement is new, even if based on Patek Philippe’s self-winding cal. 324. Designated the cal. 26-330, this 50-jewel movement is only 4.82 millimeters thick even with the weekly calendar mechanism (1.52 millimeters’ thick) piggybacking on it, allowing the watch case itself to remain slim and, therefore, dressy. The cal. 26-330 also gets Patek Philippe’s silicon parts. It spins at 28,800vph and has a 45-hour power reserve.    

Besides its innovative weekly calendar complication and calibre, the Calatrava Weekly Calendar is a sophisticated-looking watch, stunning but restrained at the same time. Its silvery opaline dial may have five hands, but each is distinct enough so there’s no confusing which indication each one is for. The hours and minutes are Dauphine-style in blackened 18-karat white gold (as are the hour indices). The rhodiumed week-number and day hands may both have red lacquered hammer heads, but these are markedly different in length. The rhodiumed second hand is slim.

But the single element that gives the Calatrava Weekly Calendar its distinct looks is its totally bespoke typography; Patek Philippe took the handwriting of one of its designers and turned this into the font for the watch’s dial inscriptions. The brand correctly points out this lends the watch a poetic touch, harking back to an era when people actually scribbled notes on paper.

Well, the Calatrava Weekly Calendar’s case itself is peculiar for a Patek Philippe — it’s made of steel. Measuring 40 millimeters wide, it has a bezel that droops over the lugs. Patek Philippe says this design was inspired by a one-off piece belonging in its museum’s collection. In fact, even the Calatrava Weekly Calendar’s reference number — 5212 — is simply a rearranged version of the one-off watch’s, which reads 2512. Of course, this one-off piece does not have a weekly calendar complication, let alone one designed to consider years with a leap week.   

Which, by the way, 2020 has.