delay the sending of email in php

Hi everyone,

I am working on a site, that will allow dentist, doctors and the like to set up automated E-Mail reminders for their clients, instead of having to call them. 

I am relatively comfortable in php.  But I have never done anything with E-Mail before...  My main question has to do with sending E-Mail on a specific date.  I have seen a few ways to do it, but nothing that really seems desirable.  One way, seemed to require that someone be using the site, to send the E-mail.  Ie: if an E- mail is supposed to be sent at 6, but no one logs on until 7:30.  Then that E-mail won't be sent until 7:30.   The other method that I have seen used cron, this second method, seems superior but much more complicated.  What method should I use???

Any, hits/tips to get me on the right direction would be appreciated!

Thanks,

Josh