It cant be greater than 10pm AND less than 8am.
It has to be greater than 10pm OR less than 8am.

Code:
IF ($away) {
  if (($asctime(HH:nn:ss) >= 22:00:00) || ($asctime(HH:nn:ss) <= 08:00:00)) { 
    .msg $nick special message 
    } | else {
    .msg $nick normal message
  }
}

* code untested *