from 1st July 1990 or from the date on which the dwelling-house is occupied by the owner, whichever is the later date, to 31st December 2010; and
(b)
from 1st July 1990 or from the date of commencement of ownership, whichever is the later date, to 31st December 2010 in the case of flats built or sold by the Government or a public authority.