from my opinion, there is no problem if credit added for future and client use it now ;Thining over this some more:
Would you only have a use-case for adding credit with a past date, or would you also be adding credit to a future date. If we add a date field, and then you add credit dated 10 days in the future, the client would still have access to that credit today, without a major overhaul of the credit system.