adding the date is as easy as adding either $date or $fulldate into the line that you're writing. For example you could change
write kbans.txt $nick banned $banmask $iif($bnick,$+($chr(40),$bnick,$chr(41))) on #
to
write kbans.txt $nick banned $banmask $iif($bnick,$+($chr(40),$bnick,$chr(41))) on # (Date: $fulldate $+ )