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

print can have options like this:
print -u2 message
And option are recognized by the leading dash. So "print -3" looks like an erroneous option is being specified. "print -- -3" is the solution. -- means no more options follow.

The "--" option for print allows you to print strings starting with a "-".

e.g. without --
with --
Print -r (raw print) works too but also ignores escape sequences.

cheers