Monthly convention.
Monthly convention. Under the monthly convention, each variation is deemed to occur for purposes of this section either:
(A) In the case of a variation occurring on the 1st through the 15th day of a calendar month, at the end of the last day of the immediately preceding calendar month; or
(B) In the case of a variation occurring on the 16th through the last day of a calendar month, at the end of the last day of that calendar month.