You want the reminder to be sent on an absolute date, and not on a date that is relative to the time when the task was assigned.
I'm not sure if this is the best way, but you could create a subprocess that sends an email, and has as input values all the pertinent information (date the email should be sent, task ID, user ID, URL to the task, etc.). The subprocess uses the Email service to send an email reminder.
Make the render process of the form variable that is used with the Assign Task operation call the subprocess.
It's not a 'real' reminder, but it accomplishes the same effect.
hope that helps
scott