Jepp ATC AU-1001 para 3.11
A pilot shall endeavour to leave the holding point on time, but one minute ahead of time is acceptable.
About does it for me, as there is no other reference to time tolerances for RTA's.
However, I do see both sides of the fence during flight when given an RTA, and have no argument with the A/C arriving 30 secs late, due lack of reference material in the Jepp.