How accounting periods work
Think of the accounting periods as brackets enclosing bookings. The periods can be completely different. The only condition is that they are all contiguous and that there is no gap. Excessively long fiscal years are just as possible as shorter ones.
If the duration of an accounting period changes, a new one must be created and the old one deleted. The duration cannot be adjusted.
When you close an accounting period, all the documents and entries it contains are fixed and cannot be changed. Apart from the entries for profit posting, etc., not much else actually happens - very simple.