Day off is a technical term; not to be confused with off duty rest. Usually 'day off' is at home base and forms part of the working schedule. A few days on the beach somewhere are not usually 'days off' but caused by the airline's schedule. If there is no flight to bring home the next day then you stay there. There are other rules about acclimatising to local time. This might be necessary before making a long flight back home. Much will depend on local agreements. These acclimatisation days are not 'off'. I'm sure BALPA have it stitched up; but things have changed drastically in recent years.