I agree that ACTUAL times of departures are measured from Off Blocks time; ie when the park brake is released and the aircraft starts moving on its wheels, be it by pushback or its own thrust. This also tallys with park brake release on ACARS equipped aircraft.
NOT when doors are closed.
NOT when the jetty is pulled away.
NOT when engine start is requested.
....and certainly not "rounded off to the nearest five minutes to make it easier to add up the blocks time at the end of the sector." This is particularly annoying and seems to be yet another lazy practice infecting our standards. I once started engines on a taxi-straight-off stand, then had to wait 5 minutes for the taxi clearance to be issued. The FO recorded it as an on time departure because we were "ready" to taxi on time. It wasn't, we were 5 minutes late releasing the park brake. Hence we recorded a delay code for Ramp Congestion.
As for the SCHEDULED time of departure, well I can't help you on that one, but I have always assumed it to be Off Blocks as above.