The authority shall have 15 calendar days after receipt of an invoice by the designated payment office to notify the contractor of (a) defects in the delivered goods, property, or services, (b) defects in the invoice, or (c) suspected improprieties of any kind; and the existence of such defects or improprieties shall prevent the commencement of the time period specified in section 4701.5 of this Part. When the authority fails to notify a contractor of such defects or suspected improprieties within 15 calendar days of receiving the invoice, the number of days allowed for payment of the corrected proper invoice will be reduced by the number of days between the 15th day and the day that notification was transmitted to the contractor. If the authority, in such situations, fails to provide reasonable grounds for its contention that a defect or impropriety exists, the date by which contract payment must be made in order for the authority not to become liable for interest payments shall be calculated from the date of receipt of an invoice.