function date_diff
{
integer _dt1=$(date_to_num ${1:?})
integer _dt2=$(date_to_num ${2:?})
integer _ndt=0
if [[ $_dt2 -gt $_dt1 ]]
then
((_ndt=_dt2-_dt1))
fi
print -- $_ndt
}


What does double dash '--' indicate ?
Can anyone please answer this