I got one last september.  After about 2 weeks I noticed it would be late by 15 to 30 minutes.  I'd reset.  A few days or a week later same thing.  I left it in the sun for 8 hours to charge.  Same thing happened a few days later.  This is a complicated watch.  I have read and reread the manual.  There is nothing I am not understanding about operating the watch.  You should be able to charge it once and leave it run.  It get plenty of sun.
Exchanged it for another of the same model.  Same exact thing is now happening after about 3 months.  Thought I got a good one but guess not.  If two of these do the same thing for one guy, I would seriously suspect the quality of this watch.  I've never had any problem with a decent quality watch before this.