When you date your ex out of desperation, boredom, guilt, or displaced regret.
I thought you and Cindy broke up a long time ago.
Yeah, we're just backdating.
Oh, cool, that sounds like a healthy life decision.
Making plans to hang out with someone you didn’t really like but had sex with on a first date, in an attempt to retroactively render yourself less of a slut/player.
"Yeah, we did it on the first date. But we’ve been backdating, so I can still say I’ve never slept with anyone I went out with less than three times … "