Interest on service tax dues

One of my clinet have not charged service tax from the service recipient.  The client did not declared his turnover cum tax in his ST3.  The client intends to charge the service tax from the service recipient at a later date.. pl clarify the date from when the interest has to be paid for the service rendered.  Is it the date of payment of the original service or the date on which service tax charged from the service tax recipient..