Judging by the reviews here, some of these watches must be OK - but not mine.
There were no instructions, but only three buttons and I can normally work out how to operate devices, so I tried to do that. I wasted a lot of time, then emailed the seller, who emailed back with instructions.
The instructions refer to buttons A, B & C, but there is no diagram. It would be easy to work out which was which if they functioned as they should, but they didn't on mine and so I wasted a lot more time, then opened it up, removed, then reinstalled battery to reset. It worked after that but the Start/Stop button required enormous pressure each time to make it work.
It is in the bin now.
If the seller is reading this could you please send me £30 for my wasted time.