SECTION 19. Assessments; extension of time for payment

The aldermen of a city or the sewer commissioners, selectmen or road commissioners of a town may extend the time for the payment of such assessments upon land which is not built upon until it is built upon or for a fixed time; but interest at the rate of four per cent per annum shall be paid annually upon the assessment from the time it was made, and the assessment shall be paid within three months after such land is built upon or at the expiration of such fixed time.