A+ Custom Requirement

I'm writing a function that needs to check the date and time clicked on/selected by the user when scheduling an appointment with Appointments+.
Essentially, I need access to the Unix timestamp created when a user clicks an available time in functions.php.
I need to get this timestamp into a variable so I can perform a few validations against it. Basic requirements are:

1. Get the Unix timestamp generated when a user selects a time into a PHP variable.
2. Know which filter the function needs to be added to. I believe it is "app_pre_confirmation_reply", but I'm not certain