A bill is payable at a determinable future time within the meaning of this Act which is expressed to be payable:
(a) at a fixed period after date or sight; or
(b) on or at a fixed period after the occurrence of a specified event which is certain to happen, though the time of happening may be uncertain.
An instrument expressed to be payable on a contingency is not a bill, and the happening of the event does not cure the defect.