955 CMR, § 2.04
If the first installment date is more than one month after the date of an agreement, an extension charge may be made and collected not exceeding one-thirtieth of the portion of the precomputed interest charge applicable to the first installment period of one month for each day that the first installment date is deferred beyond one month. Such extension charge may be collected at the time of payment of the first installment or at any time thereafter. If the first installment date is less than one month after the date of the agreement, an amount not less than one-thirtieth of the portion of the precomputed interest charge applicable to the first installment period of one month shall be credited against the precomputed interest charge on the date of the agreement for each day the first installment is less than one month.
955 CMR, § 2.04