简单的时间判断问题

function isOver(ds1, ds2){
  var a1 = ds1.split(" ");
  var a2 = a1[0].split("-");
  var dt1 = Date.parse(a2[1] + "-" + a2[2] + "-" + a2[0] + " " + a1[1]);
  a1 = ds2.split(" ");
  a2 = a1[0].split("-");
  var dt2 = Date.parse(a2[1] + "-" + a2[2] + "-" + a2[0] + " " + a1[1]);
  return (dt2 <= dt1);
}alert(isOver("2007-06-30 23:59:59","2007-7-1 0:0:0"));