... instead of resulting in error because of roll-over not being set.

Fallout from

    commit 6d424f07
    CommitDate: Tue May 2 23:12:34 2017 +0200

        Replace mouth-painted "inaccurate around leap year" rollover algorithm

that does stricter checking but DateAdd() needs a lax checking
with truncate to last day of month.

Change-Id: I9d6f95ad3ac38257d492019bd621070491e98e76