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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.

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  • $\begingroup$ you don't want to deal with even in software code -- let alone mechanical parts ... thankfuly not all people are like that $\endgroup$
    – jsotola
    Commented Jun 30, 2021 at 22:54

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

enter image description here

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.

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    $\begingroup$ My answer would have included the statement that these watches were engineered for people who took some responsibility to know what day it is. $\endgroup$
    – fred_dot_u
    Commented Jun 30, 2021 at 21:56
  • $\begingroup$ Sorry, but you don't mention a single word about how the watch knows which year or month it is? $\endgroup$
    – Dhairya W
    Commented Jun 30, 2021 at 22:03
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    $\begingroup$ It is ofcourse possible to make a mechanical program thst can track years months and wether or not month has 28 29 30 or 31 days. All you need is one or 2 more drums. But obviously its simpler if you dont do that. $\endgroup$
    – joojaa
    Commented Jun 30, 2021 at 22:47
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    $\begingroup$ @joojaa You can do a lot more than that. The Patek Phillipe Calibre 89 watch mechanically tracks the date up to centuries, including the 100-year and 400-year adjustments to leap years. But it only has a four-digit display for the year, so it won't work properly after 9999AD :( $\endgroup$
    – alephzero
    Commented Jun 30, 2021 at 23:02
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    $\begingroup$ Yes, that's what I mean. By "if required" I mean that 1st Jan, Feb, Apr, Jun, Aug, and Nov will not require adjustment because the previous month has 31 days. $\endgroup$
    – Transistor
    Commented Jul 1, 2021 at 0:03

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