Most usage products inherit month-end because accounting software inherited month-end. That is a reason. It is not the only reason.
Calendar months
A calendar month is easy to explain and hard to meter. February is short. A seat that starts on the 28th is not one-thirtieth of a month unless you said so.
If you bill calendar months, print the day count on the stub. Do not let the customer invent a 30-day month for you.
Usage windows
Some API products close every seven days because the feed is weekly. Some close when a job batch finishes. Both are legitimate. Both must be named on the book.
Scrip will not invent a window. You write period.kind and period.close. The statement header repeats it.
Late feeds
If the warehouse is late, the book stays open. Closing on a guess is how you mint credit memos. A late line is either a correction stub in the next period or a reopen. Pick one rule per book. Print it.
