The issue depends upon whether the date is a material element of the cheque. If it is so, then the issuance of a post-dated cheque is an attempt to make the cheque conditional, which contradicts the basic definition of the cheque, as quoted by Clubman above.
In fact, the Bills of Exchange Act 1882, also quoted above does not resolve the matter.
In terms of day-to-day banking, a banker seldom examines the date on a cheque, and will not lose out in court if he pays a post-dated cheque. However, if the banker is about to bounce a cheque, he will examine it, and will normally bounce it for the reason which does least damage to the reputation of the drawer. So, if you see a cheque bouncing as "post dated", it is very likely that the real but hidden reason for bouncing is lack of funds.