You might try adding a helper column that pre-groups your times into the
"bins" that you want. For example, assuming your times are in Column A, this
formula will group them as you suggest (1:00 AM - 8:00 AM, 8:00 AM - 5:00 PM,
5:00 PM - 12:00):
=IF(AND(A2>0,A2<=0.33333333),1,IF(AND(A2>0.333333,A2<=0.7083333),2,3))
It worked on my test case. You get a 1, 2 or 3 depending on which "bin" the
time falls into. In case you're wondering, 0.3333333 is the fraction of a
full day represented by "8:00 AM", and ">0" means just after midnight.
Once you have your helper column, you can use it in your pivot table to
count times that fall into each "bin".
HTH,
Eric