The in-service date starts the clock on the warranty which is transferrable to subsequent owners in the U.S. (other markets probably have different rules). The in-service date is when an i3 was first leased or sold. A Carfax report for our 2021 i3 shows that date as when the odometer reading was reported to the state, 16 May 2021, so the warranty expires on 16 May 2025.
This is the 3rd i3 we've owned since buying a 2014 i3 new 10 years ago. None of our i3's has had a problem that required a dealer repair other than 1 NHTSA recall and 3 BMW service actions for our 2014 i3, the first i3 model, all of which were performed at no cost to us. The reasons for this recall and these service actions did not affect our use of our i3 but were merely preventative actions to correct design problems.
As a result, we have never considered buying an extended warranty due to its relatively high cost and the problems that can occur when a claim is made, especially with an EV. It might make more sense to create an interest-bearing repair fund funded by the extended warranty cost.