The cost for a title in a period is based on whole months. In cases where subscription or usage is not a whole month, perhaps that should be rounded "up".

Example: if subscription goes from May 20th to June 1st (inclusive) then that would be two months rather than 0 because regular period difference would be 11 days.

Example: subscription with start = end (inclusive) - 1 day - will correspond to 1 month.

There's also a special case where a subscription start = end, and perhaps that should be considered a month as well.