var totalMin = 0;



function clearTotal(){

  totalMin = 0;

  document.mytimecalc.totaltime.value = "0 hrs : 0 min";

}



function zeroTime(){

  document.mytimecalc.myHours.selectedIndex = 0;

  document.mytimecalc.myMinutes.selectedIndex = 0;

}



function addTime(){

  var h = document.mytimecalc.myHours[document.mytimecalc.myHours.selectedIndex].value;

  var m = document.mytimecalc.myMinutes[document.mytimecalc.myMinutes.selectedIndex].value;

  h -= 0;m -= 0;

  h *= 60;

  totalMin = totalMin + h + m;

  updateTotal();

}



function subTime(){

  var h = document.mytimecalc.myHours[document.mytimecalc.myHours.selectedIndex].value;

  var m = document.mytimecalc.myMinutes[document.mytimecalc.myMinutes.selectedIndex].value;

  h -= 0;m -= 0;

  h *= 60;

  totalMin = totalMin - h - m;

  updateTotal();

}



function updateTotal(){

 var h = Math.floor(totalMin / 60);

 var m = (totalMin % 60);

 document.mytimecalc.totaltime.value = h + " hrs : " + m + " min";

}