In an age of smartphones, tablets, and personal computers with a nearly unlimited number of applications to choose from, a perpetual calendar may not even be on your radar. There are plenty of calendar applications to choose from for your smart device. However, if you are looking for something that is unique and different to add to your office space, the perpetual calendar might be something you want to acquire. This tool has been around for centuries in various forms and has not lost is practicality. What is a perpetual calendar capable of? Keep reading to find out.

With so many of us tapping our screens, we might not know the answer to the question, "what is a perpetual calendar?" The full answer goes beyond a simple definition. It dates back several centuries and involves using mathematics to take advantage of patterns that exist within the years to create a useful algorithm for computing the date. We want to help you answer the question so you can be a little more knowledgeable on this convenient invention that has been around a lot longer than your smartphone's calendar app.

What Is A Perpetual Calendar?

watch with perpetual calendar

Image via Flickr

Let's dive into the question of what is a perpetual calendar. As indicated by the word “perpetual” these are calendars that go on forever. Well, not until the end of time, that is a little too far into the future, but they do go on to a point that is beyond the human lifespan. And that in itself can be very practical for someone looking to have a calendar that they can use throughout their life.

The first recorded use of a perpetual calendar was seen in the manuscript GNM 3227a which dates back to 1389 A.D. The calendar covers the years 1390-1495. It was a tabular calendar that became popular during the 15th century. Obviously, back then it was not as easy to pull out your smartphone and tap your calendar app to find out the current date or future dates. In an age when printing was tedious and expensive, it was not as easy to produce the cute dog and cat wall, desk, and other calendars the populate modern-day office walls. Instead, perpetual calendars easily became popular tools for determining the date during earlier times.

These calendars work from algorithms that take advantage of the patterns within the year and the cycles of years that occur over centuries. These patterns can be recorded into an algorithm (or process of steps to produce a calculation) which then allows for the computation of the day of the week given a month and year. These are often recorded in tables to easily present and organize the calendar.

Algorithms And Patterns

These calendars allow for the calculation of the day of the week given a specified date in the future. For the Gregorian and Julian calendars, there are two variations. The first variation consists of fourteen one-year calendars. It has seven years for each common year and seven years for each leap year, with each of the seven years starting on a different day of the week.

The second variation consists of seven one-month calendars. These can be of the 31-day or 28-31-day length months. A table is then given to show which calendar is used given the year. Both of these variations do not include special holidays such as Easter. This calculation is done by using the Tropical Year and Lunar cycles to find the date of Easter. Similar holidays follow specific calculations that vary year-by-year.

Perpetual calendars use an algorithm to calculate the day of the week given the month and the year. These algorithms can easily be performed by modern computing software but they are often too tedious and difficult to be performed mentally. Most perpetual calendars hide these algorithms within tables. These tables are used for the fourteen yearly calendars that are available for use.

The Gregorian Calendar expresses its 400-year grand cycle as follows: 303 common years and 97 leap years. This totals 146,097 days and 20,871 weeks. Breaking this down into 100-year periods with 25 leap years, we can see how the algorithm makes use of patterns within the years to create the perpetual calendar. For the 400-year grand cycle, there are is one 100-year period with 25 leap years each. It contains 36,525 days or one day less than 5,218 full weeks. The remaining three 100-year periods with 24 leap years consist of 36,524 days or two days less than 5,218 full weeks.

A common year will begin and end on the same day of the week so the following year begins on the successive day of the week. A leap year has an extra day so the following year begins on the second day of the week that the leap year began. Every four years, the starting day advances five days, so over a 28-year period, it advances 35 days and returns both the leap year and common year progressions back to their starting weekday. This cycle repeats itself three times in 84 years with the remaining 16 years accounting for leap years and thus completing the cycle of the century.

The same patterns that are used to create the algorithms for the perpetual calendar are the ones that we use in our algorithms for creating calendar apps. This ingeniously useful pattern, first used in 1389, has followed date checkers throughout the centuries and now is commonplace in our world in the age of smartphones and apps. This makes it extremely useful and versatile, so do not discount the value of this creation because it was created in an age of antiquity. It still has a place in today's modern society.

Forms And Variants

perpetual ring calendar

Image via Flickr

Even with the proliferation of smartphones, different forms of the perpetual calendar still exist today. They come in the form of paperweights for your desk that use a twisting or turning mechanism to calculate the date. They can also be seen in the form of small pocket calendars that are in the form of a round disk. This makes them a unique item to have that has a different charm than the latest smartphone calendar app. There is beauty in its simplicity.

Traditionally, a perpetual calendar takes the form of what we described above, but there are some variants out there in the world that serve an equally valuable purpose. For example, offices and business often have a stamp that contains a rotating list of the months, days (1-31), and the year. These can be used to stamp important documents with the date. While not as complex as the actual perpetual calendar, it does keep on going and allow the user to have any desired future date for use.

We have all been in the local gas station or convenience store and seen the variant of the perpetual calendar that calculates the birthdate of someone who can legally buy alcohol. These calendars simply subtract the legal drinking age from the current date to display the birthdate that needs to be displayed on a person's identification to be able to purchase alcohol.

In watchmaking, a perpetual calendar can refer to the mechanism that displays the month and day on the clock's face. Unlike the more common annual calendar that simply displays a list of the months and the days (1-31), the mechanism used to create the perpetual calendar is similar to the calendar we described above. The mechanism takes into account the varying lengths of the months, including leap days, so the date can be displayed perpetually on the clock's face.


perpetual calendar on watch

Image via Flickr

Answering the question of what is a perpetual calendar has let us introduce you to a little history and some mathematical topics. The perpetual calendar was once a tool of necessity but it has not become an antique. It first became popular in the 15th century as a set of tables used to compute the date given the month and year. Before the age of smart devices, this was a hugely popular tool and even today it is useful. Perpetual calendars can come in many forms and designs to decorate your office or business space so that you will never be left asking what the date is when you need it most.

Furthermore, the patterns that dictate the algorithms of the perpetual calendar are the same ones that are used to create the algorithms that exist within the calendar app of your smartphone. It may not be your primary choice for a calendar but it can certainly add a fun element to have around.

We hope you have a better understanding of what is a perpetual calendar, how it uses patterns for its calculations, and different forms you might see out in the world. And we hope you have a better appreciation for calendar applications that make use of the patterns of the calendar to give you a convenient way to check the date no matter what year it. It can be the perfect item for those looking to add a special something to their office space as it is both unique and practical.

Featured Image: Flickr