DateTime::DateTime(DateTimeInitSystem) had initialized Date and
Time separately, which causes a slight possibility that it could
get a wrong datetime with almost 24 hours delay when it went beyond
midnight. E.g., the date part was of the previous day while the
time part was 00:00:00.xxx of the next day.

This also reduces duplicate code by sharing GetSystemDateTime().

Change-Id: I352d90f468f5cbc70e7936a337bed97365baa06c
Reviewed-on: https://gerrit.libreoffice.org/53612Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Eike Rathke <erack@redhat.com>