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Editorial - Not Everyone’s Calendar

Editorial Not Everyone’s Calendar

The ISO week date — ever heard of it?

Last week, I had the privilege to speak before the venerable Horological Society of New York, partners of our sister title GMT Magazine. My good friend Christian Selmoni from Vacheron Constantin had invited me to join him as co-lecturer on the subject of the calendar watches of the 267-year-old Genevoise maison, and I believe a good time was had by all. Don’t give me that dubious look — calendar watches can be absolutely thrilling as a lecture subject. Through my preparations for the talk, one particular calendar format came to my notice — one that is rarely represented in timepieces, yet underpins much of society’s administrative activity, whether in commerce or government.

Not Everyone’s Calendar

Patek Philippe ref. 5212 Calatrava Weekly Calendar © WorldTempus/Joy Corthésy

I’m referring of course to the ordinal week display, officially known as the ISO week date. Not the most attractive or poetic name amongst calendars, although this is perhaps to be expected from an entity called the International Organisation for Standardisation. Basically, the weeks are numbered from 1 to 52 (or 53, in certain years) and the month of January roughly corresponds to the weeks 1 to 4, while the month of December corresponds to weeks 48 to 52 (or 49 to 53).

Here’s where the confusing part begins. In other words, the following part is why we aren't familiar with how the ISO week date system works. The ISO 8601 standard that defines this system stipulates that the week begins on Monday and ends on Sunday. This much is fine; we’re more or less used to thinking about the week in this way. However, January 1 doesn’t always fall on a Monday. How is week 1 determined then? Do we use the first Monday of the Gregorian year in order to mark the start of week 1? What happens if January 1 is a Tuesday, meaning the first Monday of the year isn’t for another six days? What week would we be in during those six undefined days? Existential crises have been provoked by far less.

Not Everyone’s Calendar

Royal Oak Perpetual Calendar with Week Indication and Astronomical Moon © Audemars Piguet

The brainiacs at the International Organisation for Standardisation have therefore set the following rules for defining week 1. We begin by determining the first Thursday to appear in the Gregorian year. That Thursday is then designated the fourth day of week 1, and the Monday that takes place three days before will mark the start of week 1.

Therefore, if January 1 is a Thursday, week 1 actually starts on December 29 of the preceding year, since that is the Monday “belonging” to the first Thursday of the new Gregorian year. If January 1 is a Friday, week 1 will start on January 4, since that is the Monday that “belongs” to the first Thursday of the new Gregorian year (falling on January 7). The pre-week-1 days in the new Gregorian year are then considered part of the preceding year’s final week.

Not Everyone’s Calendar

Senator Calendar Week © Glashütte Original

It’s a good thing we now have computers to calculate all of this automatically for us, but if you still like a mechanical representation of the ordinal week, there are a handful of companies making watches with a week display. The Patek Philippe ref. 5212 Calatrava Weekly Calendar, introduced in 2019, is one. So is the Audemars Piguet Royak Oak Perpetual Calendar, the version that was introduced in 2015 with a revised calendar module. Glashütte Original had the Senator Calendar Week for a while, but sadly it never really caught on in the same way its other calendar watches did, and is no longer in production.

All over the world, business plans, academic terms and government budgets continue to be mapped out via the ISO week date system, since it represents a convenient system of scheduling activities during the international work week (ie, Monday to Friday). Who would like to see the display of the ISO week date appearing more frequently in mechanical watches? Let me know before the start of week 9, 2022.

Lecture 3 Comment(s)

3 March 2022
Alain Wermelinger
C’est clairement une complication qui mÉrite d’être exploitée par les horlogers. Elle est utilisée par tellement de Gens.
23 February 2022
Olivier Nebout
Bonjour, Article très intéressant et travaillant dans la planification de chantier, l'affichage du calendrier ISO est très importante donc je suis intéressé. D'ailleurs dommage que l'on ne puisse pas ouvrir une page sur la patek pour avoir plus de détails directement à partir de cette page. Bonne semaine 8 (et les suivantes aussi bien évidemment)
21 February 2022
Anne-Isabelle Rausa - Dupasquier
Merci pour cet eclairage sur le calendrier iso et pour la mise en luMiere de cette caracteristique sur ceRtains de ces bijoux de creativite portes au poignet

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