You basically have three conditions to check:

a) If start and end our both outside
b) If start is inside
c) If end is inside

Select count(*) as howmany from t1, t2 ;
where (t1.start .LT. t2.start and t1.end .GT. t2.end) ;
or (t1.start >= t2.start and t1.start <= t2.end) ;
or (t1.end >= t2.start and t1.end <= t2.end)

Costas