So if I say to someone "Meet me at 6:00" and they say "OK, I'll be there" and then turn up at 6:01, how much am I entitled to?
Promises aren't legally binding unless it's clear that both parties intended them to be.
"The law presumes that social agreements are not intended to be legally binding."
Of course, if there are special circumstances the law might take a different view. For example, if you promised to let someone move in with you and he sold his apartment only for you to then renege on that promise, he might be able to sue you.
United States Law might be different, I'm basing this on the good old UK justice system.