Countdown to pre-determined date

I have perhaps a similar question regarding Timing and Countdowns with PHPRunner 7.1 and the Calendar template.
Searching the forums I can't find this as an already answered question.
lynxgeeknyc didn't give enough details about what "it" is here .. > I need to be able to Select date and once you do, it would count down to that date.

I presume for my case I need CRON but I am checking to see if someone has created a PHPRunner "plugin".
My case is needing to send a specific reminder email, say, 15 days before the Appointment date contained in each record in the Appointment table.
As soon as an appointment and its date is submitted I would like for the Reminder Email to be sent at a specified number of days before the appointment date. I can think of a function to automatically calculate the Mail Out Date and store it in its own field as the Appointment date is selected and submitted.
Has someone here set up something similar where a function only "wakes up" once a day at a preset time, collects matching dates from fields, and triggers an email for each record with a server date that matches the "mail today" field? Integrated, I guess, with CRON.
Thank you for your suggestions.

Check this for inspiration:

http://www.asprunner.com/forums/topic/14793-sample-script-to-send-email-on-timely-fashion/

PERFECT!
This takes care of my problem.
Thank you Jane and Sergey!