18.—(1) The payment period begins on the first day of the month following that in which it is notified to the person by the Secretary of State.E+W+S
(2) Where the payment period would (apart from this sub-paragraph) end on a day other than the last day of a month, the payment period ends with the last day of the month in which it would otherwise end.
(3) If after the start of the payment period there is an interval of more than 30 days during which the person is not in pensionable employment or paying additional contributions under regulation C9 of TPR 1997 or regulation 19—
(a)the interval is not part of the payment period, but
(b)the end of the payment period is postponed by the length of the interval.
(4) If after the start of the payment period the person becomes employed in part-time pensionable employment, the length of the payment period is increased by P x (1-CS/FTCS) where—
P is the period (in years and fractions of a year) in which the person was employed in part-time employment,
CS is the persons' contributable salary for the period, and
FTCS is what would have been the person's contributable salary for the period if the employment had been full-time.