Intuitively it's tempting to say it's the moment you walk into the allocated crew room, or the rostered duty time, whichever later.
Bearing in mind that the crew room could be landside.
Imagine the case where you are rostered to take a taxi to a different base to then operate from there.
The FDP then would start as you turn up at the taxi rank (or at the time rostered to do so, whichever later etc).
All unless you had been on standby for at least 6 hrs prior.
Can of worms...