(1) In these rules, unless there is anything repugnant in the subject or context,-

(oa) "tax period" means

(i) in respect of a registered dealer who pays tax at his option at a compound rate under subsection (3) of section 16, one quarter of a year or part thereof;

(ii) in respect of a dealer, other than the dealer referred to in sub-clause (i), one English Calendar month of a year or part thereof;