Hi Dave,
Please don't use in the 'Important updates' thread as it's used for announcements etc, otherwise it would just fill up with unrelated topics!
In relation to your question - https://github.com/markn86/moodle-mod_customcert/issues/185 might interest you. You can set a date range. If this doesn't satisfy your requirements, you can create an issue on https://github.com/markn86/moodle-mod_customcert/issues with your desired functionality and i'll see what I can do, or even better, create a patch I can review!