In 1952, it was said it was ‘beyond the power of the gods … to alter the past’9. This case (at [5-15]) applies that sentiment in the context of disclaimer by the object of a discretionary trust. Private parties may agree that legal relations between them began from some date in the past. Generally, however, that will affect neither third party rights nor the operation of statutes, including tax statutes10.
Backdating an agreement will fail if it ‘undercuts the impact of a taxing act and especially if it undercuts provisions of a taxing act designed to protect the revenue’. iTip – private contracts do not generally rule over public statutes, especially tax statutes.
This case is from Episode 28 of interpretationNOW!
9 Happ  Argus LR 382 (at 384), Faraday (1927) 11 TC 565 (at 573).