Many watches are equipped with so-called date indicators. This is an indicator on the dial which indicates which date of the month it is. The date function is a commonly used feature in many wristwatches due to the practicality it offers. Instead of having to check the phone or think twice when asked what day it is, a quick glance at the wrist will tell you just that.
Apart from regular timekeeping, the date function is by far the most common complication in wristwatches.
There are seven days in a week - Monday to Friday. But the catch is that not all months of the year have the same number of days. Some months are 30 days, others are 31 or even 29.
This begs us to ask the question: How do watches know the date? And that’s exactly what we will discuss in this article
How do watches know the date?
For a digital watch, it’s easy to keep track of the date. It knows what year it is and it knows exactly how many days each month has. But with an analog watch, it’s different. How does a watch know if the month has 31 days or 29 days? And if it doesn’t, doesn’t that cause the watch to indicate the wrong date by the end of the month in some cases?
In most cases, the answer is that the watches do not know the date. And as such, they will occasionally show the wrong date. There are, however, exceptions to this rule, which we’ll discuss in detail further on.
Analog watches feature a so-called date-wheels. On this date wheel, the dates 1-31 are usually printed. This means that every month, the watch goes from 1 to 31, regardless if the month has 29 or 30 days. Every 24 hours, the date wheel rotates and changes the date. But this means that if the month has 30 days, the watch will show ”31” on the 1st and thereby needs to be reset to the correct date again.
The Gregorian calendar has 4 months that are 30 days. 7 months are 31 days long, and February is the shortest month with 28 days and 29 days in leap years.
Therefore, assuming that the watch is running at all times and doesn’t stop, a regular analog watch with a date function needs to be manually corrected 5 times a year. This is when the month has 30, or 28 days, or 29 days if it’s a leap year.
Of course, watchmakers have identified this issue and come up with various solutions. Two of these solutions are the annual calendar watches and the perpetual calendar watches. The catch is that making mechanical annual and perpetual calendar movements is extremely complex and this is why they usually come with hefty price tags. This is particularly true for perpetual calendar watches which are even more complex than annual calendar watches.
This is also the reason why, even though manufacturers have found a solution, the technology cannot be implemented into the lower-end watches. This is also the reason why a lot of people have never heard of them and wonder why their watch indicates the wrong date and how it cannot keep track of the number of days of the month.
Annual calendar watches
An annual calendar watch is a watch that displays the day, date, and month. As such, it is a much more complicated timepiece than date watches as it adds both a day and a month indicator to its set of complications.
In terms of how the watch knows the date, annual calendar watches also offer a benefit over the regular date watch. And that is that since it knows the month, it can also know how many days the month has. Therefore, it can display the date (more correctly) than the regular date watch. As a result, annual calendar watches do not have to be adjusted every month, or every other month.
The date of an annual calendar watch does, however, need to be manually corrected. But only once a year, as opposed to five times a year for a watch with only a date function.
An annual calendar watch only needs to be adjusted at the end of February as this month has 28 or 29 days. After the end of February, an annual calendar watch needs to be set to March the 1st.
Perpetual calendar watches
A watch with a perpetual calendar function is a watch that is able to know and displays the correct dates at all times. In other words, it is a watch that you don’t have to manually correct the date on towards the end of each month. This is because the watch knows exactly how many days each month has. A perpetual calendar even takes into account leap years and therefore does not have to be corrected even once a year.
Perpetual calendar watches basically never need to be manually corrected and can continue to display the correct dates for years to come. And we say ”basically”, because saying ”never” would not be quite true. A perpetual watch occasionally needs to be corrected, but only once ever 100 years. But let’s be honest, the watch will stop several times during this time, of example when it needs a service or when someone doesn’t wear the watch, so it’s not really an issue for regular everyday use anyway. The perpetual calendar watches are programmed to display the correct date up until the year 2100.
But generally speaking, we could say that a perpetual calendar watch knows the date by being programmed to know exactly how many days each month has. And therefore, it never needs to be manually corrected. It takes into account leap years, the 28-days in February, as well as distinguishes the months with 30 and 31 days.
Perpetual calendars are usually only found from luxury watchmakers like Patek Philippe, Audemars Piguet, and other haute horlogerie brands. They tend to start at a minimum of 100k EUR.
The real answer to the question of how watches know the date is that they don’t. At least not most of them. Most analog watches with date functions are equipped with date wheels that go to 31 and will continue to display the date up until 31. As such, the date needs to be manually corrected five times a year.
Annual calendar and Perpetual calendar watches solve the issue of regularly having to manually correct the date frequently and only need to be adjusted once a year, or once every 100 years respectively. As such, annual calendar and perpetual watches solve this issue, but due to their complexity, they also come with a considerably price tag. And this is also the reason why most people are not very familiar with them, or have even heard that such a thing exists.