fdo#72916 correct rounding of time part of DateTime

Change-Id: I135478755f9e5a844119129b470fef8de2cd0409
Reviewed-on: https://gerrit.libreoffice.org/7163
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>