What is the effect of a covenant requiring a mortgagee not to sell or lease a property for a month?

The following quotation from page 532 seems instructive on the point:— “In Doe v. Day, 2 Q.B. 147, freeholds and leaseholds were conveyed in mortgage with a proviso that, upon payment on the next 5th October, the conveyance should be void, but in case of non-payment it was to be lawful for the motgagee, after a month’s notice in writing demanding payment, to enter into possession, and to make leases and sell, and the mortgagee covenanted not to sell or lease until after such notice. The Court, following the authority of the passage in the Touchstone, referred to in Doe v. Lightfoot, and acceding to the doctrine of that case, came to the conclusion that, inasmuch as, after the day of payment, the time, if any, during which the mortgagor was to hold was not determinate, but altogether uncertain and as there was no affirmative covenant whatever that he should hold at all, the covenant, therefore, that the mortgagee should not sell or lease, or, even if it be construed should not enter, until a month’s notice, was a covenant only and no lease.’“