All you do is set the hour, minute and second properties of
your target date object!
var targetDate:Date = new Date(2007,5,4, 9, 30, 54); // Wed
June 4 09:30:54 GMT+0200 2007
And your code will work ;)
However i recomend you look in to using some other means of
updating the displayed time. onEnterFrame might do it too offen
making this metod a bit unpractical. Seeing how we only display the
seconds you might only need an interval timer set to 100 ms.