Ok, I know it's based on time in service, mileage, and logarithms that some engineer programmed. But I just got the 29 day warning for service A at 1,500 miles and 4 months!!

It's surprising at best; time consuming at worst. Is this a new record low mileage? I was going to go drive around to see if it resets or if the day countdown changes. Either way I'm calling the dealer to get the service light reset in 30 days if that is what happens.

Mine has 10K and 11 months old - message says " Service B" needed.
I have always thought 10K is service A also, but may be I'm wrong after reading your post.

What is the four months? The time lapsed since you bought it? If so, you probably bought it 8 months after manufacture date (perhaps), so it has been exactly a year since the manufactured date. Check the sticker on the drivers door jamb to verify. If that doesn't seem correct then call the dealer.

It follows a decision tree based on driving habits, temperatures and other factors and determines which service is needed. On my older R class I had seen the indicator ask for service B for three days and then it changed to C. The dealer ignores these and goes by mileage only.