In this Part--
"relevant date" means--
(a) in relation to an existing use referred to in section 4.65(a) of the Act--the date on which an environmental planning instrument having the effect of prohibiting the existing use first comes into force, or
(b) in relation to an existing use referred to in section 4.65(b) of the Act--the date when the building, work or land being used for the existing use was first erected, carried out or so used.