Calculate Time OVERNIGHT

I have a form that successfully calculates time passed if it is on the same day (ex. 9am to 4pm). However, it returns "0" if going overnight (ex. 11:30pm to 4:30am). I haven't been able to find a solution for this yet. Below is the current script for total time:

var today = "11/01/2004";
var start_time = this.getField("Start Time");
var end_time = this.getField("End Time");
var starting = today + " " + start_time.value;
var ending = today + " " + end_time.value;
var c_start = util.scand("h:MM tt", starting);
var c_end = util.scand("h:MM tt", ending);
var diff = (c_end.valueOf() - c_start.valueOf()) / 1000;
var sec = diff % 60;
var min = ((diff - sec) / 60) % 60;
var hrs = ((((diff - sec) / 60) - min) / 60);

event.value = hrs + ":" + min;