[AlwynMike]

My brother has an F31 330i that was manufactured in April 2017.
It was first registered in June 2017

The iDrive says April or 16,000miles to the next service.
It's not yet done 12000 miles overall - it had an oil change etc last year.
Obviously, it will need an MOT in June.

What he doesn't want is 2 journeys for service and the attendant hassle with transport.

Does the 3 year manufacturing date count down per day, or is it to the end of the month (April in his case)?

To keep up a nice green service record, I've advised him not to wait until May or June to get the 3rd year service done (this may only be a brake fluid replace). His local dealer Sytner often have a 6 week waiting list for a Courtesy car so he wants to get something organised soon! May be get the service done on 30th April, with MOT at the same time which will come out as 30th May as it's not due.