You stated that a mechanical watch will be unaffected as an absolute fact.
1. Have you been there and conducted experiments to prove absolutely that mechanical watches will be unaffected?
2. Are you a geologist or have knowledge of the geological composition where the tunnel is located?
It is entirely possible that the material surrounding that tunnel can indeed affect electronic and mechanical devices, especially if the area is heavy with a mineral such as magnetite.
We needn't go to such extremes in order to make a decisive conclusion.
(a) Every single report about this tunnel that you can find only references cellphone clocks being set back. There is no mention of, for example, the digital clocks inside the cars themselves being affected. Additionally, one can presume that
somebody in China wears a watch, but not a single thing mentions that wristwatches are being affected.
Seeing as that would make the stories more exciting, if there were something to report on these items, they certainly would be mentioned.
Therefore, it is safe to surmise that only cellphones are affected.
(b) The cellphones are only being set an hour back; never any other interval. This strongly corroborates the idea that cellphones are simply being directed to a different tower when going through the tunnel if they update while passing through.
Based on these two points, it is very safe to conclude that mechanical watches will not be affected by passing through the tunnel. They do not update using the information from a cell tower. Mystery solved.
Moz