calendar TBO

One finds many 20+ year old aircraft advertised with engine time =
airframe time, both in the neighborhood of 1000-1600 hours. Now, it is
my undersatanding that aircraft engines have a calendar TBO as well as a
tach TBO. however, prices for these aircraft do not seem to reflect
engines that are beyond TBO. Is calendar TBO something that is generally
ignored? Should a prospective buyer make a hard negotiating point about
it?
-- 
Dan
C172RG at BFM