Trigger email to customer 1 day before activity

OK, so I've been trying to set up an auto email to customers to remind them of an appointment that's coming up. I find that I can only trigger the notification at the moment the activity is created or modified, which isn't exactly what I need to happen, so I'm hoping to find some help here. Let me lay out the process:

A salesperson creates an activity to meet with a customer at a specific date / time.
-there are fields for the date / time, customer info, etc., status of activity - which at this point is "scheduled"

I'd like an email to be generated to the customer. I've created fields for the link the customer gets and clicks to switch to "confirmed" status, and that's all working great. The issue is that if the appointment is for, say, 2 weeks out, I'd rather the email not go out until 1-2 days before as a reminder. Unfortunately, I can't seem to get that to function; the email either goes out immediately or not at all, no matter how I set the conditions. Any ideas on what I might be missing?



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Thanks,
Chris Newsome
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