Check the Accuracy of your Watch
Disclaimer: I am not responsible for any damage to your watch from following these procedures.
Measuring your watch’s performance against an accurate time signal is a great to make sure your mechanical watch is operating within spec and running well. Remember to check your watch’s accuracy over at least a week long period. This will tell you how stable your watch is in keeping time. A watch that gains or loses a lot of time in the same position and level of winding will need service.
Get an accurate time signal, I prefer the US Naval Observatory: http://tycho.usno.navy.mil/simpletime.html
“Hack” your watch at 12o’clock or 0 seconds, meaning stop the main seconds hand. In most watches this means pulling the crown out all the way. In some watches, you have to apply backward pressure lightly to the crown in the time setting position to stop the seconds hand like in the Speedmaster Professional.
You can synchronize the watch now by pushing in the crown directly when your time signal reaches the zero point. Make sure you have your minutes set 1 ahead of your time signal.
If you watch has a date function, my preferred method of insuring AM/PM synchronization is to set the date 1 day before your current date by the quick date set. Now after you’ve done that, forward the time on your watch until the date clicks over to the current date and then you can set the time insuring your AM/PM is synchronized. You can now hack the watch.
I recommend you write down on a piece of paper the time and date you’ve synchronized your watch. 24 hours later, you can see what your deviation is from the atomic clock. If you record this time again in 24 hour periods you can tell if you have a stable watch or if it will need adjustment by a watchmaker.
For a modern mechanical watch based on an ETA movement, anything more or less than 15 seconds is probably a good sign it needs some looking into. For a Japanese or Chinese movement, 40 seconds is a good guide but some may even go over 90 seconds and be within spec. Remember even with a 15 second deviation, your watch is still more than 99.9% accurate.