In a remarkable first for the world of watchmaking, the dual calendar invented by Vacheron Constantin is expanded into a full mechanism combining two distinct functions that can be viewed simultaneously —the traditional Gregorian perpetual calendar with 12 months, 52 weeks and seven days, and the ISO 8601 standardized business calendar.

Gregorian perpetual calendar Gregorian day name Gregorian month name Gregorian retrograde date Leap year and four-year cycle display

The Gregorian perpetual calendar, which automatically takes into account leap years and the number of days in the month, is displayed on the Reference 57260 watch with a retrograde hand for the date, dials for the day and month and an aperture positioned to the right of the retrograde date for numerals 1 to 4 of the cycle of leap years.

In modern times the most commonly used calendar is the Gregorian tropical solar calendar, named after its founder pope Gregory XIII who reformed the Julian calendar that had been in use in Europe until that point.
Introduced by Julius Caesar in the year 46 B.C., the Julian calendar had since fallen out of phase with the seasons, meaning that Easter no longer coincided with the Spring Equinox, as tradition dictated. A practical result of the Gregorian reform was that the number of leap years was reduced from 100 to 97 in every group of 400 years. Each year divisible by 4 is a leap year with the exception of years divisible by 100; however, these century years are leap years if divisible by 400. For example, the years 1700, 1800 and 1900 were not leap years, whereas the year 2000 was.
The adoption of the Gregorian calendar corrected the anomalies of the old Julian calendar and provided a system that indicates the seasons precisely and goes from January 1st to December 31st in 365 days and 12 months. In leap years an extra day is added to the month of February, a phenomenon known as intercalation.
The Gregorian calendar is now in everyday use in most regions of the world and considered to be the standard calendar. However, as it is solar, its dates do not indicate the moon phases.

Number of the day of the week (ISO 8601 calendar) Week to view (ISO 8601 calendar)

This is the very first time that the ISO 8601 calendar has become a watchmaking function. Its creation and incorporation as one half of the dual calendar not only required extremely complex calculations, but also a sense for the practical that was honed by Vacheron Constantin watchmakers as they endeavored to transpose theory into a workable, easy to read function.

The ISO 8601 business calendar is a standard created by the International Organization for Standardization for use in the field of international finance, for example for determining the financial year for a company, paying salaries or rents on a weekly basis, planning projects by week, etc. The purpose of this standard is to provide an unambiguous method of representing dates and times, especially for the transfer of data between countries with different writing conventions. For the time, the ISO standard uses the 24-hour clock with a time zone designator where needed.
If the ISO 8601 method is used, the week-of-year and day-of-week calendars take priority over the month and date of the traditional calendar: the week number is displayed on the dial concentric to the month display and the day number —from 1 (Monday) to 7 (Sunday)— in an aperture positioned just above the week dial.

The ISO system has a complete 400-year cycle and works in groups of seven days, with the week starting on a Monday. However, an ISO year can comprise 52 or 53 complete weeks, 53 when New Year's Day falls on a Thursday (Thursday or Friday for leap years), which happens every five or six years. In the ISO calendar, Week 1 comprises the first Thursday of the year and inevitably January 4th whereas the last week comprises the last Thursday and inevitably December 28th. This system obliges the user to adopt a new form of interpretation, for example: if the traditional calendar displays Thursday, September 17th, the ISO calendar will display 4 in the day aperture (Thursday being the fourth day) and 38 on the week dial.

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