Fix (hopefully for the last time) problems with datetime values displaying

Fix (hopefully for the last time) problems with datetime values displaying
like '23:59:60' because of fractional-second roundoff problems.  Trying
to control this upstream of the actual display code was hopeless; the right
way is to explicitly round fractional seconds in the display code and then
refigure the results if the fraction rounds up to 1.  Per bug #1927.