The DLR website says: One-day tickets that are purchased this year are valid now through December 30, 2021. So, if you purchased your tickets now, they would still be good for quite a while. If the price were to go up between now and the day you use them, you would just need to pay the difference at the ticket booths. You could wait until the website updates the dates and prices or you could buy now and see if the prices change by the time of your visit. Either way will work.