Md. Code, State Finance and Procurement § 15–104

(a) Except as provided in § 15–105 of this subtitle, interest shall accrue at the rate of 9% per annum on any amount that:

(1) is due and payable by law and under the written procurement contract; and

(2) remains unpaid more than 37 days after a unit receives an invoice.

(b) Interest shall accrue beginning on the 31st day after:

(1) the day on which payment becomes due under a procurement contract; or

(2) if later, the day on which the unit receives an invoice.