The planned duty is anything before your report (i.e. what is on your roster). Once reported anything that happens is what is actually happening, the captain may then consider the use of discretion. In this case your roster and the schedule (the plan) was 11:45. You report and are given the flight plan which states that the flight time is over 12 hours (the actual). It is then permissable for the captain to choose to use discretion.
Points to note though, it is not sensible or good practise to regularly roster duties that are close to max FDP as this results in captains frequently having to consider the use of discretion. Secondly you mention the flightplan time was 12:12 but the FDP does not finish until the aircraft is on stand. The best that you could manage is 10 min to start up and taxi out and probably another 10 minutes to taxi in. This gives a total FDP 13:32 (including 1 hour report) or had you already taken that into account.