To be safe, I’ve already booked my medical renewal for a date 1 month before my present certificate expiry date, so I’ll find out when I go.
An early renewal is the only way I can see of ensuring that you have a valid certificate for the full duration of a trip offshore. By rights, a company can’t really send you offshore for a 4 week trip, when your medical certificate only has 1 week of validity left.
It just means that your medical renewal date will be forever moving forward by a few weeks, if no time is carried over.