Hempy just looked at that reference in my Jepps, ATC AU 505 5.3 and the statement is correct.
The application though is very vague. Many moons ago when starting my training I had been lead to beleive that the 30 sec was primarily in reference to the watch on my wrist (or time keeping device in the aircraft).
So lets say ATC's clock expects us at Arbey at 0210 and their clock is +/- 30 sec they could expect me from 0209.5 - 0210.5.
Say my clock is consistently 30 sec different from ATC then they could expect me from 0209 - 0210 or 0210-0211.
My maths suggest they are looking at a possible 60 sec error. So with combined errors (ATC 30 sec and Pilot 30sec) could +/- 60 seconds be more correct ?