The obvious way to keep a car's mileage current is to read it continuously — a dongle in the diagnostic port, or the phone's location. Both work. Both mean a household app now holds a minute-by-minute record of where a family goes.
Two readings is enough
Two odometer readings and the dates between them give a rate. The rate, applied forward, gives a usable estimate for today. It is less precise than telemetry and it is precise enough for the only question being asked: is a service near, or not.
The tuning is all about not overreacting
A weighted average that leans on recent readings adapts to a genuine change in driving. A clamp on how far any one gap can move the rate stops a single road trip from redefining normal. Together they behave sensibly on the two cases that actually occur: someone changes jobs, and someone drives to the coast.
The estimate is labeled as an estimate, every time it is shown. That is not a caveat; it is the feature.