Also acclimatised means 3 local nights away from base in a time zome more than 2 hours wide.
Basically you have to do more than a 3 days trip (4 days or more) LH going to east coast US or Carrib to be acclimatised.
If you don't (which a lot of the LGW trips are just 3 days) then you come back to the UK acclimatised still.
If you come back unacclimatised and have to operate then your hours of operation are restricted (you can't operate without 18 hours rest and limits are 1 hour less than scheme restrictions or more).
As tofster said as well. If you do end up not having your second day off you do get it in Lieu and you may take it later in the month at a mutuallty agreeable time and it will not be included to your monthly total of 9 days off.