How much does COSC mean to you?

[FK77]

To me it means a level of accuracy. Usually with the movements they are finished to higher tolerances. Non-COSC movements (from some manufacturers) may also be better than COSC, but most will not be as stable, or as capable of as good accuracy over a given period as a better movement, or a movement with better parts as standard.

If a movement has a stated <25 secs per day and you get better than that great, but don’t expect it. At least with the big brands they will regulate your watch under warranty and get it within their stated figures.

This. Most of my mechanicals perform at either +1 or +2 spd under normal conditions and when off the wrist. However, only the COSC ones (and that just means Rolex in my case) are capable of maintaining that +1 or +2 spd even after some rather sporty use.

I took my GMT to RSC a few years ago when it was running at +6 s/d and they happily regulated it under warranty, despite being (just) within COSC parameters because they expected +/- 2 from it. On the other hand, I had to be certain that my 60th Anniversary Speedmaster was running at +13 s/d i.e. at least 1 sec outside the stated tolerance before I could take it to Omega for regulation.

Having said all of that, I don’t think I wouldn’t buy a watch that I like just because it wasn’t certified.

my Hamilton, which is not COSC still keeps time to COSC

My Hamilton Khaki keeps better time than COSC, in fact it’s more accurate than any of my Rolex watches and runs at just +1 spd under normal use.