Again thanks for your thoughts, and I am considering the
possibilities of incorporating each of these ideas from all who have
responded. The latest problem  I discovered is that some of the 'bad
data' errors include a null value (like when the system cannot
interpret the input) in the source, which get's outputed
as "00:00:00" for the dispatch value--which get's interpreted as
midnight, exactly.
So, using your formula would interpret a 'null' entry, which I
had hoped to kick to the error  List, as a legit entry, so it would
undo the previous  error traps.

With all that everyone has offered, though, my latest
attempt/thought has been to have a there supervisor make a judgement
on the time  of the dispatch. If they listen to the call  (which are
all recorded, since this is a 911 center) and find  that the call's
dispatch time is legit and in fact it was dispatched exactly at
midnight "00:00:00", or a few seconds  later, then with a button,
that time will be "honored" in the calculations. Otherwise, I can't
see anyway to filter the calculations, without undoing the whole
purpose of the error check.

One thing for sure, though, all the ideas which everyone can
kick out will be welcome to me, indeed, with thanks. I've said it
many times, but this is a great group and I read the postings every
day.