What you can do is set up a script that performs a find for all records where one field indicates that the notice has not yet been sent and the date is X days older than the date returned by Get ( CurrentDate ).
If your file is hosted from FileMaker Server, a schedule can run such a script once a night.
If it is not, you might use fileOptions to set up a script to run each time you open or each time that you close the file.
There are also ways to use a scheduling utility such as Windows Scheduled Tasks to run this script on a schedule.
For examples of scripts that perform finds, see: Scripted Find Examples