[Greg9999]

I don't own a Rolex to keep perfect time, if I wanted that I would wear my Casio G-Shock quartz with "atomic" timekeeping.

However, I was surprised after a recent regulation of the accuracy of my GMT Master II 16710. They got it to +0.454s per day (+60.5 sec after 128.2 days or 4.3 months). This is with the watch worn normally, no winding, and no laying it down in any of the positions to speed up or slow it down.

This is a 2007 watch with 3185 movement. so the COSC is -4/+6 sec/day. As you know, Rolex went to a COSC of +/- 2sec/day later on.

I did not use a timegrapher, but rather a simple (but highly recommended) iOS app "Watch Tracker". It relies on your accuracy of pressing a button when the second hand sweeps over the hour marker, but it's going to be pretty accurate over time.

I made sure the watch was not magnetized, the amplitude is 239 - 280 depending on the position of the watch, and considering 270 - 310 is a good range, it's hanging in there. The watch had a one full servicing in 2017.

Not bad for 28,800 vibrations per hour.
How about your experience? Comments welcome.