Calculate Difference in hours between Start and End Date/Times in mm/dd/yy h:MM tt format

I am creating a fillable form for our employees to request vacation time off.  I have pieced together calculations scripts from various forums but can not get the result I want. Say an employee wants to take 01/04/19 off.  They would enter a start date/time of 01/04/19 8:00 am and end date/time of 01/04/19 5:00 pm.  I want the form to calculate the difference in hours.  Now the trick is that if they take more than 4 hours (leave after 12 noon) the deduction needs to deduct an hour from the result, to account for the lunch hour so that employee is only charged 8 hours of vacation rather than 9.  Here is the script I have pieced together, please advise:

function Date2Num(cFormat, cString) {

// convert cString with cFormat to number of minutes from Epoch date

// convert to date time object

var oDate = util.scand(cFormat, cString);

// convert date time object to minutes

return Math.floor(oDate.getTime() / (1000 * 60));

} // end Date2Num function

// format for input date & time

var cDateFormat = "mm/dd/yy h:MM tt";

// field names

var cStartField = "Start DateTimeRow1";

var cEndField = "End DateTimeRow1";

// get field values

var cStart = this.getField(cStartField).value;

var cEnd = this.getField(cEndField).value;

// clear the result value

event.value = "";

// compute the difference in minutes if there is data

  if(cStart !== "" && cEnd !== "") {

if (cEnd > 12) {

nDiff = (Date2Num(cDateFormat, cEnd)-60) -  Date2Num(cDateFormat, cStart);

} else {

nDiff = Date2Num(cDateFormat, cEnd) -  Date2Num(cDateFormat, cStart);

}

}

// convert to hours

event.value = nDiff / 60;