It is a fundamental principle that leases must have a fixed minimum duration and have a definite starting date; otherwise they are void (Say v Smith (1563) Plowd. 269). So, for example, a lease ‘for the duration of the war’ would be void (Lace v Chantler  KB 368).
This principle was confirmed by the House of Lords in Prudential Assurance Co Ltd v London Residuary Body ( 2 AC 386). An agreement made in 1930 provided that ‘the tenancy shall continue until .. the land is required by the Council for the purposes of the widening of Walworth Road.’ The landlords claimed that this purported lease was void because of the uncertainty of its term. The House of Lords, with Lord Templeman giving the leading judgment, upheld the first instance decision that the lease was void.
The rest of the House of Lords agreed but associated themselves with this passage from the judgment of Lord Browne-Wilkinson:
‘This bizarre outcome results from the application of an ancient and technical rule of law which requires the maximum duration of a term of years to be ascertainable from the outset. No one has produced any satisfactory rationale for the genesis of this rule. No one has been able to point to any useful purpose that it serves at the present day.’ (at 396)