Perpetual Calendar

A 50-year pocket perpetual calendar

A perpetual calendar is not a specific type of lunar or solar calendar, but rather a calendar which gives the exact day of the week on which a day falls over a period of many years. In this way it allows an individual to, for instance, look backwards to the 2010s and learn what date the second Monday of a particular month might have been, or to look forwards to the 2030s and see what day of the week the first Friday of a month might be and so on and so forth. Perpetual calendars were introduced in the eighteenth century as a means of looking up dates quickly over a period of five, ten, fifteen or so years for business or personal reasons. In decades gone by, one was most likely to come across a perpetual calendar on a watch or clock. Today they are found in one form of another on many digital devices such as laptops and smartphones. Most perpetual calendars operate according to the Gregorian Calendar, the most widely used solar calendar in the world today.

Perpetual calendars historical contextPerpetual calendars historical context

In centuries gone by individuals had very little information about what day of the week a particular date might have fallen on many years earlier. This was just as well in some sense, but the prevalence of different calendar systems, such as the Julian calendar, Gregorian calendar, Orthodox calendar and Coptic calendar across Europe made it difficult to establish uniformity around dates and years anyway. However, by the middle of the eighteenth century and particularly after Britain’s adoption of it in 1752, the Gregorian Calendar came to be ubiquitous across most of Europe and further afield in the Americas.

Thomas Mudge

As this occurred, it became reasonable to have a universal calendar which could tell a person quickly what day of the week a date would fall on in times to come. This was good for business purposes. For instance, in an age before computers, if a banker was giving out a loan to a merchant in London in the 1770s he might want to have a means of looking up quickly the day of the week that the loan or interest payments thereon might fall due three or four years later. This led individuals in financial institutions and other businesses to begin using analog paper calendars which had the days of the week for a period of years worked out on them. But a smaller, more digital method was soon in the works.

Such business and personal necessities led the English horologist and watchmaker, Thomas Mudge, to invent the first perpetual calendar in 1762. This was inbuilt into a watch with a new element to it which Mudge had earlier invented, the detached lever escapement, which subsequently became integral to all modern mechanized watches. Using this, Mudge was able to develop a series of displays on a timepiece which could be moved used a lever escapement to work out dates several years in the past or in the future. Such perpetual calendars were limited to being able to calculate the day of the week over a period of four years when they were first created, but as timepieces and wristwatches developed through makers like Patek Phillipe in the nineteenth century they became more advanced.

Perpetual calendar detailsPerpetual calendar details

The perpetual calendar, as noted, is not a specific type of calendar. They usually mirror the more commonly used Gregorian Calendar, but they are employed primarily to tell the day of the week in years past or years to come. As Patek Philippe and other watchmakers developed their timepieces they were able to introduce innovations in the late nineteenth and twentieth centuries which allowed the calendar to determine the day of the week for years and decades to come. The only major discrepancy concerning them is that they have to be manually adjusted three times every 400 years to take account of the fact that the Gregorian Calendar skips a leap year three times every 400 years. Thus, if you have a time piece with an in-built perpetual calendar today and if you are hoping to make it to the year 2100, you will have to adjust it that year, the next year in which the leap year is skipped.

Relevance of perpetual calendars in modern timesRelevance of perpetual calendars in modern times

Perpetual Calendars are in in widespread use around the world today. While they were initially produced as analog calendars or as timepieces which might contain pages charting the year over a period of five, ten to twenty years, today they are most often associated with digital devices. For instance, on the average smartphone today an individual can look up what day of the week a particular date will be years or even decades from now. While this function is found in a basic form in most phones, more detailed and useful perpetual calendars which also include the dates on which certain religious or civil festivals like Easter or bank holidays will fall can be downloaded as apps to one’s phone. These include Perpetual Calendar and Endless Calendar and can often calculate the day of the week a day fell on for a period of up to 10,000 years. This makes these new types of digital perpetual calendars of utility for a broad range of functions, including historical research.

Explore more about calendarsExplore more about calendars

• Understanding Historical Dates and Calendars for Historical Research by James Tanner, article on the MyHeritage Knowledge Base
• Understanding Dates: Five Common Mistakes to Avoid on the MyHeritage Blog

References

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