Mont. Code Ann. § 20-9-411

In order that the dates of payment of installments on school district bond issues may coincide as nearly as possible with the largest monthly tax collections, all school district bonds shall preferably bear a date of some day in June or December. For this reason, the bonds may be dated back not more than 5 months from the time of the actual sale, but no interest shall be charged on these bonds before they have been delivered to the purchaser and payment has been made by the purchaser. Interest accrued on such bonds according to their terms at the time of delivery shall either be refunded by the purchaser or deducted from the first interest payments. The failure to date such bonds in June or December shall not affect their validity.