p.boxster
643
30/31st of a month
I can really understand how annoying this feels. A brand new watch not working properly.
But reading your issue there MIGHT be an explanation as you own the watch just for a few days before the change of a month with 30 days to the next month. Exactly for this case, the manual instructs a special way adjusting the watch which is to be followed IN DETAIL. ("if you wish to set the date to the 30th or the 31st , proceed as follows...")
I had the same issue with a 5146G not changing the month, that´s why I know.
I suggest the following just to check:
Set the time to 6am like the manual instructs for setting date.
Then switch the date to lets say the 15 th;
then switch the month to Nov; then set the date to 29.
Now move the time by hand till day and date change to 30 and then to 1. The month should change. This procedure won´t harm movement or anything and may readjust the calendar.
If this doesn´t work, only the AD will be able to help...
Best wishes!!