Our modern calendar follows somewhat complex rules, with some months have 30 days, some 31 days, and febuary varying between 28 and 29 days depending on if it’s a leap year or not. So even though our calendar is complex, mechanical watch calendars are very simple, leading to occasional incorrect dates on your watch. It’s extremely difficult to handle the complex computations of our calendar within the tiny space of a mechanical watch.
The most common, and most simple watch calendar, takes the approach that all months have 31 days. Meaning that the date on the watch rolls ahead from one to 31, and then stars over at 1, regardless of the month. This simple approach means you have to manually roll forward the date to the correct
The only down side to this greatly simplified approach is that five times a year, you have to manually roll the date forward to get the correct number.
There are perpetual calendar watches that can handle this complexity. But since they are more complex mechanisms, they are also more complex to set, and far more intricate and difficult to make and maintain. That’s why most manufactures opt to go with the simpler version to reduce the cost of their timepieces and make it easier for their customers to own and maintain.
Digital watches, of course, have no problem with this, since our complex calendar is simple in the digital realm.