Timesheet help

I have a simplet time sheet: Date, Start time, End time, total hours. I am having issues with 8am-8pm being a 12 hour shift. It shows up as 0 hours and everything after that it increases one at a time. (ie 8am - 9pm = 1hour). Here is what I'm using. 

// start
var start = this.getField("1").value;
var startArr = start.split(":");

// minute
var minutesEnteredS = startArr[1].substring(0,2);

var startHour = startArr[0];

// finish
var finish = this.getField("2").value;
var finishArr = finish.split(":");

// minute
var minutesEnteredF = finishArr[1].substring(0,2);

var finishHour = finishArr[0];

// difference
var hourDiff = (finishHour - startHour);
if (hourDiff < 0)(hourDiff = hourDiff + 12)
var minDiff = ((minutesEnteredF - minutesEnteredS) / 60);

var output = hourDiff + minDiff;
event.value = output;