Subject to the provisions of this Part of this Act and any regulations thereunder, a driving licence shall have effect -
(a) in the case of a licence which is expressed as not having effect until it is signed by the grantee and which is not signed by him before the commencement of the period for which it is granted the part (if any) of the period for which it is granted which is subsequent to its having been signed by the grantee, and
(b) in any other case the period for which the licence is granted.