24 hr time format is not giving correct values for the time difference calculation

I have the following code for my pdf time sheet Time In , Time Out in 24 hr time format. When I use 8:30 AM -10:00 AM it give correct value as 1.5. But when I use  11:30 am - 12:15 AM I am expecting 0.75 but I am NOT getting that value. Please advise and help me out here.

var start1 = this.getField("Recall1_Wk1_Thurs_TimeIn").valueAsString;
var finish1 = this.getField("Recall1_Wk1_Thurs_TimeOut").valueAsString;
var start2 = this.getField("Recall2_Wk1_Thurs_TimeIn").valueAsString;
var finish2 = this.getField("Recall2_Wk1_Thurs_TimeOut").valueAsString;

var totalTime = 0;

if (start1!="" && finish1!="") {
var startArr = start1.split(":");
var finishArr = finish1.split(":");
var hourDiff = Math.abs(finishArr[0] - startArr[0]);
var minDiff = Math.floor((Math.abs(finishArr[1] - startArr[1]) / 60)*60);
totalTime+=(hourDiff*60)+minDiff;
}

if (start2!="" && finish2!="") {
var startArr = start2.split(":");
var finishArr = finish2.split(":");
var hourDiff = Math.abs(finishArr[0] - startArr[0]);
var minDiff = Math.floor((Math.abs(finishArr[1] - startArr[1]) / 60)*60);
totalTime+=(hourDiff*60)+minDiff;
}

var totalTimeHours = Math.floor(totalTime/60);
var totalTimeMinutes = (totalTime-(totalTimeHours*60))/60;
var output = totalTimeHours+totalTimeMinutes;
event.value = output;