I recently purchased an analog chess clock. That is, although battery powered, it has two normal clock faces, including the red flags, but also including red second hands.
The second hands can be set to the zero (12 o'clock) position by letting the clock run until they each arrive there, and then setting the minute hand to the desired position. In other words, the clock can start with the second hands pointing vertically.
Two problems arise:
The minute hands, used to signify the end of the match when they arrive at the 12 o'clock position, are difficult to set precisely. For one thing, there is a bit of "slop" in the setting knob, and for another thing parallax errors can make the hand seem to move slightly if you rotate the clock as you look at it.
Now, at the end of a (say) 5-minute match you could conclude the match ends when the flag falls, or the second hand reaches the 12 o'clock position. In practice these do not occur simultaneously, with around +/- 10 or 15 seconds being observed. In this case, when is "time up"? When the flag falls, or when the second hand reaches the vertical?
If the answer is "when the flag falls" — regardless of where the second hand is — isn't that a bit unfair? What if you clearly see that there are 15 seconds to go, based on the position of the second hand?
In a similar vein, having observed that setting the minute hand precisely is difficult, is there not some unfairness in using analog clocks at all? If (deliberately or inadvertently) one player get an extra 15 seconds, and the other player gets 15 seconds less, then that could make quite a big difference in a 5 minute game.
Also, is there some convention about who sets the clock? Does one player set it and the other check it? Does each player set their own side? Does a third party set the clock?
In response to a comment requesting more information:
I got the clock from eBay
This is what it looks like:
It appears to be a PQ9905 model
Here is the "user manual" with links to the manufacturer's web page: