datediff = function(date1, date2, interval) {
  var delta = 1;
  switch(interval) {
  case "day":
    delta /= 24;
  case "hour":
    delta /= 60;
  case "minute":
    delta /= 60;
  case "second":
    delta /= 1000;
  case "millisecond":
    delta *= date2.getTime() - date1.getTime();
  }
  return Math.round(delta);
};

var diff = datediff(new Date("2012-04-28T14:30:00Z"), new Date("2012-04-29T14:30:00Z"), "day"); 
for (var i = 0; i < 50; i++) {
  diff = datediff(new Date("2012-04-28T17:00:00Z"), new Date("2012-04-28T17:30:00Z"), "minute"); 
  assertEq(diff, 30);
}