How to calculate time differance using AM/PM format?

How to calculate time differance using AM/PM format?

Hi Guys
I am trying to calculate the time differance in minutes between 2 time

so i have 2 time fields  09:20 PM and 10:00 PM  i want to return 40 minutes


here is what i have done so far (the output is always NaN)

  1.             function calculateTime() {
                    var start = $('#Report_startTime').val();  //09:20 PM
                    var end = $('#Report_endTime').val();     //10:00 PM


                    var start_time = Date.parse(start, "hh:mm tt");  //Convert Time from A/PM to time
                    var end_time = Date.parse(end, "hh:mm tt"); //Convert Time from A/PM to time

                    var startDate = new Date("1/1/1900 " + start_time);
                    var endDate = new Date("1/1/1900 " + end_time);
                    var difftime = end_time - start_time; //diff in milliseconds
                    console.log(difftime);
                }