Date.parse function should work with that format , if is not working you can use the Array.split to get the values you need,
var d:Date=new Date();
var matches:Array=string.split("/");
d.setUTCFullYear(int(matches[0]), int(matches[1]) - 1, int(matches[2]))
To Set the time you can use the method setUTCHours
d.setUTCHours(0,0,0,0)
EDIT:
var string:String="2008/11/13";
var d:Date=new Date();
var matches:Array=string.split("/");
d.setUTCFullYear(int(matches[0]), int(matches[1]) - 1, int(matches[2]))
trace(d);
d.setUTCHours(12,11,13);
trace(d);