8 years end on the day you took delivery plus eight years. Example: delivery August 4,2012. Warranty is good til August 3, 2020. If something happens on August 4, 2020 that would be up for discussion. My guess: the warranty expires on August 3.
My extended warranty on the roadster actually expired a couple days after it had a cooling problem which resulted in exchanging the PEM. Had this happened just a few days later, I would have had to eat the cost. Lucky me.
If any problems occur during the warranty period make sure to report them. The repair will be covered by warranty even if the fix cannot be completed before the warranty expires.