How can (many) mechanical watches keep track of the current day of the month when these vary so much over months and years?

Question

Having dealt with dates and time in programming context for many years, I have realized what a nightmare our common calendars are, and how much they vary by the year/month/etc.

Yet there are fully mechanical watches which have a little "day number" window saying something like "1" (for the 1st day of the current month) or "27" (for the 27th day of the current month).

How is this possible? Not only do the number of days vary between the months, but it seems much more complicated than that, with certain years having more days and all that nightmarish stuff you don't want to deal with even in software code -- let alone mechanical parts!

Have I misunderstood something fundamental about the calendar system in common use today? Are these watches ultra-complex inside with all sorts of cogwheels dedicated solely to keep track of these differences? How does the watch know (or how can it be told) what the current year (or even month) is, so that it can display the correct day?

Most of the mechanical watches I've seen only display the current day number -- not the month and the year. So this really confuses me in multiple ways.

Answer 1

You advance the date manually, if required. It can be done without disturbing the time setting.

Figure 1. Image source: Watches under 500.

Position I – for hand-winding of the main spring.

Position II – position of the crown in which you will able to change the day and date. If your watch only has a date window, you can change the date by turning the crown clockwise. In this case, turning the crown in opposite direction (counterclockwise) does nothing. If your watch has a day and date windows (as the one shown on the picture), turning the crown clockwise will change the date, and turning the crown counterclockwise will change the day.

Position III – you should pull the crown to that position in order to be able to set the time.

The watch has no idea what the true date is or what month or year it is and at midnight on the 30th June will advance to the 31 (which is an invalid date). It will be up to the user to advance it manually to 1 (1st July). The day of the week, once set, will keep track as it's just running a seven day cycle.

1st Jan, Feb, Apr, Jun, Aug, and Nov will not require adjustment because the previous month has 31 days.