If you've purchased Anytime Return tickets you can use the return portion on any train within one calendar month of outward travel. Any specific date shown refers solely to your seat reservations, which you can forego without invalidating the tickets for travel on any other train.
If you've purchased Off Peak Return tickets, the same applies except that you must ensure that your return journey is made on any service classed as 'off peak' by the train operator.
However if (as seems most likely) you've purchased separate 'Advance' single tickets (for travel in either direction), those tickets are only valid when you occupy the correct reserved seats on the designated train service. Travel at any other time, on any other date, or in any other seats is not permitted.
Train operators rigorously enforce their rules regarding ticket validities. If they didn't, people who found that all the cheap tickets had already been sold for their preferred service could simply book for the following week and board the 'wrong' train, thus bypassing the ticket quotas.
Chris