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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.

Dhairya W
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  • `you don't want to deal with even in software code -- let alone mechanical parts` ... thankfuly not all people are like that – jsotola Jun 30 '21 at 22:54

1 Answers1

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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.

Transistor
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    My answer would have included the statement that these watches were engineered for people who took some responsibility to know what day it is. – fred_dot_u Jun 30 '21 at 21:56
  • Sorry, but you don't mention a single word about how the watch knows which year or month it is? – Dhairya W Jun 30 '21 at 22:03
  • I covered that in the first sentence. I thought it was fairly obvious that the watch has no idea what the true date is and tonight will advance to the 31st (June). It will be up to the user to advance it manually to 1st (July). The day of the week, once set, will keep track as it's just running a seven day cycle. – Transistor Jun 30 '21 at 22:20
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    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. – joojaa Jun 30 '21 at 22:47
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    @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 :( – alephzero Jun 30 '21 at 23:02
  • @alephzero yeah sure, no problem, more or larger mechanical programming elements needed. Now all one really needs is to incorporate leap seconds into the mix ;) it is however likely that a person living in 10000 ad would not mix up with years 10000 earlier. But who knows immortals among us might need reminding. I mean im regularily off by a year. What is to say a immortal ciuld not be of by 10 millenia – joojaa Jun 30 '21 at 23:08
  • Wait... By "You advance the date manually, if required.", you actually mean that you have to keep manual track of what day it is, in spite of your watch having this (apparent) feature, and constantly set it manually? As soon as you reach day 28, you have to manually remember/look up what month it is and change it either tomorrow or in a few days? Is that really what you mean? If so, what do you mean by "if required"? – Dhairya W Jun 30 '21 at 23:14
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    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. – Transistor Jul 01 '21 at 00:03
  • Plus it is common practice to pull the stem out and stop the wristwatch at night or when not wearing it. You then have to set everything each time you put it on. That's what the regulator clock on the mantle is for. – Phil Sweet Jul 01 '21 at 01:26
  • Yep. In high school with my analog watch I would correct the day every second month at the end of the month. – DKNguyen Jul 01 '21 at 01:34