B767-300 departure from IAD refused ATC takeoff clearance behind B777 with less than 2 minute interval.

Narrative

Was ordered to take off with less than the minimum 2 minute separation time; behind a B777 (hacked time at his takeoff roll and takeoff thrust application) and was at XA05 when Dulles Tower cleared us for 'immediate takeoff.' Stated that 'I could not do that for about another minute.' Tower became very agitated and told us to either start roll 'Now' or 'Get off the runway.' Told him we did not have 2 minutes for about 30 seconds and would NOT take off without 2 FULL minutes. By the time this little conversation had run its course; we had the 2 minutes; and took off. It would be nice if we all played and honored the same time-proven rules of safety (i.e.; 2 full minutes wake turbulence separation...minimum). That was a full 777 going to Europe -- with no wind. It was bad enough as it was; with 2 minutes. We were kicked around pretty good. Someone needs to come out to IAD and re-train our IAD Controllers on wake turbulence separation times -- and the time starts NOT when you clear the prior 777 for takeoff -- but when he actually starts his takeoff roll (he sat there for about 10-20 seconds after he was cleared).