Code for Setting Time

Code for Setting Time

Hi, everyone.  I'm looking for what should be a simple script that manipulates a time/date field.  Specifically, I want to send out appointment reminder text messages an hour before the appointment.  I was planning on having two time/date fields.  Appointment Field and Reminder Field.  

I need a script that will " SET Reminder Field TO Appointment Field - One Hour".  

Can anyone help me?