Where is the interview time and date stored? During the event where you send an email, can you simple retrieve the inter interview information and include it in the email? You should be able to use the username to retrei e interview info.
I think you are going to have to create some type of scheming table to keep track of the interview schedules you propose when you send the email to the user. You have to track those dates otherwise you won't know how many people have received the proposed schedule and who has accepted. This is obviously a complication but take your application out of the picture for a moment.
How would you handle this if this was a paper based process?. You have to design the process first, then apply technology to make it efficient. You can't build an app then expect the process to adapt. This is MIS 101.
Sorry the MIS 101 is a reference to Management Information Systems, 101 referring to a basic class. When you say something is 101, you are indicating that its a basic principal or concept in that field/class/concept.