Each employer’s rate for the twelve months commencing January first of any calendar year shall be determined on the basis of the employer’s record through the preceding June thirtieth. In the event the division has been unable to calculate the rate, or the calculation of such rate by the division has not yet become final, in time to advise such employer of such rate a reasonable time before the date any contribution payment may be due, the rate in effect for the preceding calendar year shall be paid by each employer and an adjustment of any overpayment shall be permitted or additional payment demanded in the event of an underpayment, in connection with any different rate established for such calendar year, but no interest shall accrue on any such underpayment until the expiration of thirty days from the mailing of such demand.