Time Difference Calculation with an Exception

I am working on a vacation request form that has a field for start time and end time in h:mm tt format.  I have a script that is working for calculating the difference in those fields, however, I need this have an exception and I am not sure what to add/change.

If an employee request 8:00 am to 12:00 pm the script works fine, however, if they request 8:00 am to 5:00 pm it fails because it doesn't take the hour lunch period into consideration.

Please advise.

// get the values of the fields

var sStart = this.getField("Start TimeRow1").value;

var sEnd = this.getField("End TimeRow1").value;

// clear field as a default action

event.value = "";

// compute only if we have values

if(sStart != "" && sEnd != "") {

// add a formatted date value to the time field values

sStart = "1-Jan-1970 " + sStart;

sEnd = "1-Jan-1970 " + sEnd;

// convert the date time strings to date object

var oStart = util.scand("d-mmm-yyyy h:MM tt", sStart);

var oEnd = util.scand("d-mmm-yyyy h:MM tt", sEnd);

// convert date object to milliseconds since 1-Jan-1970

var nStart = oStart.getTime();

var nEnd = oEnd.getTime();

// compute the difference in milliseconds

var nDiff = nEnd - nStart;

// convert to seconds

var nSeconds = Math.floor(nDiff / 1000);

// compute the whole hours from the minutes

var nHrs = Math.floor(nSeconds / (60 * 60));

// compute the remaining minutes

var nMins = (nSeconds / 60) % 60;

// set field value string to formatted time string

event.value = nHrs + ":" + util.printf("%,202.0f", nMins);

} // end have values