I don't understand what does action period denote by future-in-the-past?
It denotes the action which referenced to the future, relative to current time moment, from the past, relative to the current time moment or It denotes the action which is future to a some past, relative to the current time moment and may be past relative to the current time moment?
For example, Future-in-the-past simple:
He said he would go to the dentist.