Time Calculation with break (Day and Night Shift) in 24hr format

Try this:

var start = this.getField("8.0").value; 
var finish = this.getField("11.0").value; 

if (start == "" || finish == "") { 
 event.value = "";} 
else { 
 var lunchout = this.getField("9.0").value; 
 var lunchin = this.getField("10.0").value; 
    
 var startArr = start.split(":").map(Number); 
 var finishArr = finish.split(":").map(Number); 
 var startHour = startArr[0]; 
 var startMin = startArr[1];
 var finishHour = finishArr[0]; 
 var finishMin = finishArr[1];

 if (finishHour < startHour || (finishHour === startHour && finishMin < startMin)) { 
  finishHour += 24;} 

 var hourWDiff = finishHour - startHour; 
 var minWDiff = (finishMin - startMin) / 60; 

 var hourLDiff = 0; 
 var minLDiff = 0; 

 if (lunchout != "" && lunchin != "") { 
  var lunchoutArr = lunchout.split(":").map(Number); 
  var lunchinArr = lunchin.split(":").map(Number); 
  var lunchOutHour = lunchoutArr[0]; 
  var lunchOutMin = lunchoutArr[1];
  var lunchInHour = lunchinArr[0]; 
  var lunchInMin = lunchinArr[1];

  if (lunchInHour < lunchOutHour || (lunchInHour === lunchOutHour && lunchInMin < lunchOutMin)) { 
   lunchInHour += 24;}

   hourLDiff = lunchInHour - lunchOutHour; 
   minLDiff = (lunchInMin - lunchOutMin) / 60; }

  var totalHrs = (hourWDiff + minWDiff) - (hourLDiff + minLDiff); 
    event.value = totalHrs;}