From what I gather (as a wet-behind-the-ears SLF), the advertised time of departure is the target push-back time. However, if an aircraft pushes back within 15 minutes of the advertised time, it's deemed to be on schedule.
Therefore, if an a/c pushes back at T+14 then sits on the tarmac for 20 minutes, it's still regarded as a punctual departure.
I hear certain British railway companies are trialling a similar scheme...
