Hello Jo.
Firstly thank you to David for the good suggestion.
The only issue with the suggestion Jo is that your certifications typically have one audience assigned to them [ i.e for all users who need to complete it ]. So when the certification was set up you had to choose a due date for all users factoring in both existing staff and future new joiners. Whilst the 'complete within one month of start date' is great for new joiners to stop them receiving 'overdue' reminders and stop them showing 'overdue' on their first day, it does mean that existing staff will have due dates based on when they started, which could have been 10+ years ago. For example, on setting up you probably wanted to make sure all staff were compliant on a certification immediately [ as they were existing staff and already should have done it ], but you want new joiners to have a different due date. The only way to achieve that is with multiple audiences [ one for existing staff and one for new joiners ]. There is no audience rule for 'is new joiner' so you'd essentially have to have an audience with the targeting rules and add in 'and start date is > than today. Then have a second audience with just the normal targeting rules and 'is not in audience xxx new joiners. That obviously creates more audience management and work.
An additional point is if you have customised reports for certification status, like a Red, Amber and Green report. I've worked with lots of trusts that specify new joiners should show up as 'red' on that report. So you may want to check your organisations requirements if you have such a report, as if the new joiner is not overdue they may appear a different colour to what you expect in any of those custom reports.
Best wishes,
Andy