Automatically Sending E-mail At Time Specified By Input From a Form

Question

So for a website, I want to have this site send an e-mail reminder to those who sign up. However, I only have figured out how to send the email as soon as the form is submitted stating that they have made a submission. I would like to be able to send a reminder at a specific date, lets say a week before the event they have signed up for. The problem is dealing with the input since it would have to be a variable. So I'm curious as to how I would go and make the site send a reminder email by taking the date of the event and storing that value but taking off a week. Then when that date hits, have it send an email stating that they have their event in one week. I'm not sure if this is possible but any help would be greatly appreciated.