It is the policy of this City that payment pursuant to any authorized, written procurement contract shall be made by the City to the contractor not later than 30 calendar days from the receipt date of a proper invoice. Payment pursuant to a construction contract shall be made by the City to the contractor not later than the date specified in the contract or, if no date is specified, then not later than 30 days from the receipt date of a proper construction estimate. City Code, 1976/83, art. 1, §27A(b) Ord. 89-280