Mine

alias brb {
  set %away $$?="Reason for being Away:"
  var %long = $$?="How long till you will be back? (In minutes)"
  set %awaytime $calc($($ctime + $calc(%long * 60),2))
  away %away
}

on *:TEXT:!status:#: {
  notice $nick $me is away, %away $+ . $me will be back in $duration($calc(%awaytime - $ctime))
}

alias back {
  unset %away
  unset %awaytime
  away
}