You demagnetize a watch by passing it through a small demagnetizer or handing it to a watchmaker, which resets the magnetized parts in seconds. Magnetism mainly affects mechanical watches, making them run fast. Most quartz watches are largely immune.
Why magnetism makes a watch run fast
A mechanical watch keeps time with a fine steel hairspring. When that spring gets magnetized, its coils cling together, the balance speeds up, and the watch gains time, sometimes minutes a day. It's not broken. It just needs demagnetizing.
Common sources of magnetism
Everyday magnets are the culprits: phone cases and stands, laptop and tablet lids, speaker magnets, magnetic clasps and jewelry, refrigerator doors, and handbag closures. Setting your watch on or near these repeatedly can magnetize it over time.
How to tell your watch is magnetized
The classic sign is a mechanical watch suddenly running noticeably fast. You can confirm it with a phone compass app: slowly bring the watch near it, and if the needle swings around, the watch is likely magnetized. This is a quick, free check.
How to fix it
The easiest fix is a cheap watch demagnetizer. Place the watch on it, press the button, and slowly lift the watch away while the field is active. That gradual withdrawal is what clears the magnetism. A watchmaker can do the same in seconds, often for little or nothing.
Why quartz watches are different
Quartz watches don't rely on a hairspring, so magnetism rarely affects their timekeeping. A very strong field can temporarily disturb one, but it usually returns to normal once away from the magnet. This is mostly a mechanical-watch problem.
How to prevent it
Keep your watch away from speakers, magnetic phone mounts, and magnetic clasps. You don't need to be paranoid, but a little distance from strong magnets goes a long way toward keeping a mechanical watch accurate.