(a) “Month” means 20 working days or four weeks of five working days each, including legal holidays, with respect to the computation and crediting of service.
(b) “Month,” for all other purposes, means a period commencing on any day of a calendar month and extending through the day preceding the corresponding day of the succeeding calendar month, if there is any such corresponding day, and if not, through the last day of the succeeding calendar month.