I've come to believe - and be comfortable with the notion - that people who say, I'll be there at 3:30, actually aim at leaving their house at that time, and of course there are many things that preclude one's leaving at any given time. Therefore, the person who planned to leave at 3:30 may easily find that it's 10 minutes before the hour before they pick up their purse (it is usually women, I'll grant you) and run because I'm so sorry to be late, but I got a phone call (or just remembered) that I had to make a phone call before I left the house. I'm so sorry.

Does that make sense? It does to me. And I am a person who errs in the other direction- I'm always early.