HPCC-12513 Clean up various windows warnings

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Thought about this after reading a comment by Richard. Don't you want to use abs instead, i.e. sqrt(x^2)? For the case that   nowdt.getSimple() > nextdt.getSimple().
Or perhaps reconsider changing to unsigned at all. I'm haven't looked into what these two are but their names may suggest that they are deltas (nextDT) in which case it is quite possible for the above condition to be true.