The warranty clock starts at purchase price. The details of the warrenty are associated with the date of manufactering. Companies won't (typically) retroactively change the warranty.
Note that the probability that you will need the warranty (or that it will apply) is very low. If there is a manufactering defect in the frame, I suspect you should find out pretty quickly.
The savings you should get on the 2008 is a certain thing. The effective value of the extended warranty is, comparitively, very, very low.