Your syntx is wrong, i have not tested it but try the following amended code

Code:
on !*:JOIN:#channel:{
  if !$($+(%,$nick,.,welcomed,$cid),2) {
    set -u43200 $+(%,$nick,.,welcomed,$cid) 1
    msg $nick 05Hi $nick $+ , this is a message that you will receive only once every 12 hours.
  }
}