For the record, re: the "back-dating" thing. Some cheating spouses lie about when their contact with OM/OW started, but more often, they lie about when they say they were "done" with the marriage, emotionally. Or at a minimum, they never adequately conveyed the depth of their unhappiness to their spouse, who then later gets blindsided by the affair.
Manipulating these two dates -- when they were emotionally "done" with the marraige, and when they first behaved inappropriately with their OP -- is the basic foundation upon which the wayward spouse will build their entire "compartmentalization" paradigm.
Put more simply, it's how they justify their destructive decision-making and behavior.
