Future-perfect-in-past tense is an absolute-relative tense that involves three points in time in the past. The tense refers to a time that is in the future, relative to another point in the past, but is in the past relative to a point in its future. All these points in time are in the past relative to the moment of utterance.
- John left for the front; by the time he should return, the field would have been burnt to stubble.
The burning of the field is in the future relative to John’s leaving, but is in the past relative to his returning. All these events are in the past relative to the moment of utterance.