The most novel complication shown on this face of the watch, however, is the perpetual calendar – which is actually two perpetually calendars. The first functions as a conventional perpetual calendar. The date is shown on a retrograde display at 12:00, while the day of the week is indicated on the left sub-dial, and the month is indicated on the right. There is also a Leap Year indication at about 1:00 on the dial.
The second perpetual calendar is mechanically linked to the first, as it’s based on it: this is a perpetual calendar which shows the date as specified by ISO 8601, which was first published in 1988. ISO 8601 specifies both a notation, and a special calendar. The notation itself can be used for the standard Gregorian calendar, and uses the format YYYY-MM-DD (with allowed variations, also specified by the ISO). The basic idea is to eliminate the possibility for ambiguity in date notations, especially between individuals in countries with differing traditional date notation systems. The ISO also specifies an alternate calendar system: a “week numbering” calendar. Such a calendar uses no months. Instead, each week of the year is given a number, and the date is given as year, week number, and number of the day of the week. The week number information is shown in the reference 57260 on the sub-dial on the right, while the day number is shown in a small window just above it.
The ISO 8601 week numbering calendar is used mainly in the financial and business worlds; its biggest advantage is that there are always a whole number of weeks in the year, and the notation is unambiguous. There are drawbacks, however, to ISO 8601. The first is that the beginning and end of the week numbering year does not always correspond exactly to January 1, as the week numbering year always begins on the first Monday after 52 full weeks. This means that at least in some years, the Gregorian date for January 1 might be in the new year of the Gregorian calendar, but in the old year in the ISO system (for instance, January 1 might be 2015 in the Gregorian calendar, but 2014 in the year numbering calendar). The other drawback is that the ISO 8601 system, in order to stay relatively aligned with the seasons and with the Gregorian calendar, needs to add a total of 71 intercalary weeks over the 400 year cycle of the Gregorian calendar. These 53 week years are called “long years” in the ISO system.
ISO year 2009, for instance, was a 53 week, rather than a 52 week year. As there appears to be no provision on the 57260 for showing a 53rd week, we assume in such years a manual adjustment must be made, and the week numbering calendar is perpetual in the sense that it always shows the correct week numbering date for the corresponding Gregorian date – we’re hoping to clarify this with Vacheron Constantin.