hours on duty?

[purple haze]

dear all

now i know that the max. duty time is eight hours for pilots, but technically when is that?

is it when you sign on or when you take off, and if there are delays, are they taken into account? (delays in the holding pool etc...)

when you sign on, what procedures take place before signing up?

are there any variations for airlines or is the same, an eg of variation, bottle to throttle for ba 8hrs ek 12 hrs.

any answers would be great.

cheers