Something

on *:TEXT:!countdown:#:{
  if ($ctime > %date) {msg $chan Countdown expired}
  else { msg $chan $duration($calc($ctime - %date))}
  }

  on *:TEXT:!setdate:#:{
    if (!$ctime($2-)) 
    {msg $nick Use !setdate <date>
    msg $nick Valid date formats are:
    msg $nick January 1 1970 00:00:00
    msg $nick 3rd August 1987 3:46pm
    msg $nick 21/4/72 1:30:37
    msg $nick Wed 1998-3-27 21:16}
    else {set %date $ctime($2)
  }
}