Date / Time question

Hello everyone-- I'm trying to validate a date to make sure that it isn't any date before the current date (i.e. today's date is 08-25-2000 and I don't want any dates before that date). Right now I have one function that checks to make sure that the actual date is a valid date, however it doesn't check if the date is before today's date. The application I am making is setting appointments and it's kind of dumb to set an appointment for yesterday. I'm not too sure how exactly to go and perform this check on the date so any help would be great! Thanks, Jay Paulson