Does the late charge clause need to be retrospective or prospective in operation?

Defendants maintain that if we determine that the late charge clause is in fact a liquidated damage provision, we should make our decision prospective in operation. But our holding herein involves none of the serious consequences to the parties and to others (see Westbrook v. Mihaly (1970) 2 Cal.3d 765, 87 Cal.Rptr. 829, 471 P.2d 487) which would warrant our making an exception to the rule that a decision is retrospective as well as prospective in operation.