Something

alias aftellen {
  tokenize $asc(:) $1-
  if ($1 isnum 0-23) {

    var %time = $+($1,:,$iif($regex($2,/^[0-5][0-9]$/),$2,00))
    var %date = $iif($ctime($date %time) > $ctime, $date, $asctime($calc( $ctime + (60*60*24)), dd/mm/yyyy))

    echo 10 -at Time remaining until %time $+ :4 $duration( $calc( $ctime( %date %time ) - $ctime )) $+ 10.

    .timercountdown 0 300 echo 10 -at Time remaining until %time $+ :4 $!duration( $!calc( $ctime( %date %time ) - $!ctime )) $+ 10.
    .timer %time 1 0 .timercountdown off
  }
  else {
    echo 4 -s Error: invalid parameters.
  }
}