Think I've managed to solve this.  In case anyone is interested here's how:

One extra audience for new joiners (this is dynamic based on a rule that takes a users start date against their profile).

I assign that audience on every certification with a relative date (one month after program enrolment).

The new joiners will be assigned certifications by two audiences, the main one (which has a fixed date in the past) and the new joiner one.  As they've been assigned through two, it will take their furthest date.  This means users who are added when the system goes live and who have never done it will be existing staff and will be set to the fixed date before go live to show as overdue, but new joiners will take the new joiner audience date in the future.