Time Elapsed Calculation HH:MM:ss

Hello Experts,

I'm a bit of a java rookie but usually I can get a code to swing out the way I need it to do. 

I have spent a week working through forums searching for a solution but as yet have not found a code that is effective.
I have a PDF which needs to calculate the time difference between two sets of time input.

Time1 = HH:MM:ss

and
Time3 = HH:MM:ss

OR (ideally)

Time1 = DD/MM/YYYY HH:MM:ssand

Time3 = DD/MM/YYYY HH:MM:ss

I have managed to make the date calculation work stand alone but ideally I would have it occur in the one calculation.

The code is reactive and only occurs with a specific drop down result.

Below is the only time calculation I have managed to get to work. It displays accurately in decimals, but does not calculate seconds yet, and try as I might when I modify the code to attempt the required format I can't get it to work. 

I am using Adobe 2017

//get whatif value

var cDisplay = this.getField("Dropdown8").value

// compute number of days only if there are values for both dates and displays

if(cDisplay != "A" & cDisplay != "C" & cDisplay != "D" & cEnd != "" & cStart != '')

//

var cDisplay = this.getField("Dropdown8").value

var strStart = this.getField("Time1").value;

var strEnd = this.getField("Time3").value;

strStart = strStart + " 1/1/70";

strEnd = strEnd + " 1/1/70";

if(cDisplay != "A" & cDisplay != "C" & cDisplay != "D" & cEnd != "" & cStart != '')

{

   var timeStart = util.scand("hh:MM ", strStart);

   var timeEnd = util.scand("hh:MM ", strEnd);

   if (timeEnd < timeStart) {

   var strEnd = this.getField("Time3").value

   var strEnd = strEnd + " 1/2/70";

   var timeEnd = util.scand("hh:MM", strEnd);

   var diff = timeEnd - timeStart;

   var oneHour = 60 * 60 * 1000;

   var timeleft = (diff/oneHour);

}

var diff = timeEnd - timeStart;

var oneHour = 60 * 60 * 1000;

var timeleft = (diff/oneHour);

//event displayed

event.value = timeleft.toFixed(2);

} else {

event.value = " ";

}