Clarifies code and gets rid of explicitly casting the operand to sal_Int32.

Also in preparation of removing DateTime::operator+=(sal_Int32) that is
confusingly similar to DateTime::operator+=(double) and just depends on type.

Change-Id: I83422e2940fbb017978db9b5734b4966228af3de
Reviewed-on: https://gerrit.libreoffice.org/40248Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>