What year is the car? I assume it is a 98 since you say 70K. However,
half way through the 98 production the newer type mechanicl timing
belts were introduced. The new ones go 105K miles. Warning: All 98
owners manuals say change at 70K miles so Volvo does not officially say
they go to 105K miles.

See (lists serial engine numbers with newer belt):

http://www.brickboard.com/AWD/?id=581137

and

http://www.brickboard.com/AWD/?id=554434

Personally, I would not wait even if you have the newer design (best to
follow Volvos recommendation since they have the real engineers who investigate
and decide these service intervals). Note that a broken timing belt will
damage a large part of the engine leaving a $3000 repair bill (at least).

If you have the older design you can do it yourself. Cost of the belt is
about $45 from volvoparts.com. http://volvospeed.com/bay13.htm has
the instructions. An independent garage should be able to do the job
for 2.5 hours labor on the older design (newermechanicl design
should probably be left to the dealer).

With financial realities being what they are....I would not exceed the recommended interval change by more then 10,000 miles. After that, it could be the biggest mistake you have ever made....