I have a 1/97 build date on my 98 S70 T5M. Purchased in
April of 97. There are laws about when you can start selling
a model year. Apparently, April of the year befoee is OK
(I don't know exactly when you can sell a model year but the
lead time is pretty close to one year).

I purchased when they had just hit the dealers in volume.

If it is any consolation, my 98 has had fewer problems then
other people have reported here. My theory: When starting
the assembly line up they run it slow and take care to really
monitor for problems. Others will say the opposite is true and
avoid an early build. Who knows (well, Volvo does).
Of course, I've still had problems but I'd say about the same
as our 95 855 (which has also been, overall, a very good car).