A midnight promise is broken by the dawn.
An empty promise, with no intention of keeping it.
Words spoken to impress or persuade an audience with nothing to support them.
He made several midnight promises, but he told his friends what his girlfriend did for him.
His midnight promise was broken, when he left town, after he found out his girlfriend was pregnant.
The custody agreement, was regarded as a midnight promise, when my ex left town with the kids, without giving notice.