Timecard with auto calculate for hours worked deducting lunch

This is what i have this far. 

Field Names:

1 - Clock In, 2 - Lunchout, 3 - Lunchin, 4 - Clock out

If you use whole hours such as 08:00, 12:00, 13:00, 17:00 - works great

but if you do this 

08:00, 12:30, 13:00, 17:00 or 08:00, 12:00, 13:30, 17:00 - error calculating. What am I missing!!! I have been workign on this hours and cant seen to fix it. Any help would be greatly appriciated.

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var start = this.getField("1").value;

var finish = this.getField("4").value;

if (start=="" || finish=="") event.value = "";

else {

    var lunchout = this.getField("2").value;

    var lunchin = this.getField("3").value;

    var startArr = start.split(":");

    var finishArr = finish.split(":");

    var hourWDiff = Math.abs(finishArr[0] - startArr[0]);

    var minWDiff = (Math.abs(finishArr[1] - startArr[1])/60*100);

    var hourLDiff = 0;

    var minLDiff = 0;

    if (lunchout!="" && lunchin!="") {

        var lunchoutArr = lunchout.split(":");

        var lunchinArr = lunchin.split(":");

        hourLDiff = Math.abs(lunchoutArr[0] - lunchinArr[0]);

        minLDiff = (Math.abs(lunchoutArr[1] - lunchinArr[1])/60*100);  

    }

    var totalHrs = hourWDiff + (minWDiff/60) + hourLDiff + (minLDiff/60);

    event.value = totalHrs;

}