For civil helicopter flying logbook flying time is calculated thus:-
From taxi for the purpose of flight to rotor shutdown.
If you taxi to pick up pax the flight time starts from when you taxi with pax onboard to rotor shutdown.
If you lift straight to the hover it is from wheels or skids airborne to rotor shutdown.
My advice is to get the license as soon as practicable but be aware at the moment a JAR ATPL is valid for 5yrs and the CAA always want a hefty wedge of loot to re-type the first page of the license on renewal.
It is also more difficult to get the license if your last tour is a ground tour.
HF