When creating a payable, you will need to set your date and due date. Depending on when you are posting the payable, this might require you to post-date the transaction.
The payable date will start off set to today's date, and the due date will be set to 30 days from today's date. In order to set your payable date for a future date, you first need to set your due date to fall after the transaction date. This is because a due date can't be set before the posted date.