Something

on *:TEXT:!back:#:{ 
  if ($($+(%,brb,.,$nick),2)) { tokenize 32 $($+(%,brb,.,$nick),2) | msg $chan $nick is now back and was away for $duration($calc($ctime - $($+($,$0),2))) $+ . | unset $+(%,brb,.,$nick) }
  else notice $nick You never went away.
}
on *:TEXT:*:#:{
  if ($1 == !brb) {
    if ($2) { set $+(%,brb,.,$nick) $remove($2-,$) $ctime | tokenize 32 $($+(%,brb,.,$nick),2) | msg $chan $nick will be right back in  $+ $($+($,1-,$calc($0 - 1)),2) $+ . }
    else notice $nick You must specify a length of absence. For example !brb 5minutes. 
  }
}