Within 72 hours after receipt of notice from a proposed excavator or from the system of a proposed excavation, but not including Saturdays, Sundays, and legal holidays, an operator shall mark the location of its underground facilities in the area of the proposed excavation. An operator and an excavator may by agreement fix a later time for the operator’s marking of its facilities. Once an operator has marked the location of its facilities in the area of an excavation, the excavator shall be responsible for maintaining the markings. An operator who participates in an underground facility damage prevention system shall not be required to locate underground facilities not owned by the operator; underground facilities which the owner shall not be required to locate may include, but shall not be limited to, facilities running from a house to a garage or other outbuilding.