Do time stamp deltas with unsigned arithmetic.

Do time stamp deltas with unsigned arithmetic.

First, check whether the originate timestamp is greater than the other
timestamp and, if so, calculate (originate - other) and print a -
instead of a + before it.  This means we're not trying to stuff the
difference between two unsigned values into a signed value of the same
width (the result of which is undefined if the difference doesn't fit in
the signed variable).