O.C.G.A. § 7-4-15: When interest runs on liquidated demands; promissory notes payable on demand.

All liquidated demands, where by agreement or otherwise the sum to be paid is fixed or certain, bear interest from the time the party shall become liable and bound to pay them; if payable on demand, they shall bear interest from the time of the demand. In case of promissory notes payable on demand, the law presumes a demand instantly and gives interest from date.