That's fair enough but if you plan to be somewhere at some time you should keep to your word. Other people go out of their way to make it happen, so should you.
I'm not into time micro management either, but I do like to make the most of mine. Specially when I have a lot to do. Depending on other people to keep their word and they don't just shows how little someone cares about my efforts. Its a pretty selfish act.