A friend once told me that they couldn't attend a late dinner because they weren't eating after 6pm in order to lose weight. I told them that while it's true that people who stop eating early in the day will generally consume less calories on an average day, studies show that the time of day that you consume your calories doesn't matter at all. After showing her the documentation, she thanked me and was embarrassed that she had just accepted it without thought.
The next time I saw her was at a late dinner. She had gained 15 lbs. Now I just keep my mouth shut.