javascipt utility that increments/decrements to the next 15 minute interval

Question

Can anyone find any bugs or performance improvements here?

<a href='#' id='from_arrow_down' onclick="timeOffset(-1,'from_time');">Click me</a>

function timeOffset(offset,time_id)
{

  // javascipt utility that increments/decrements to the next 15 minute interval.
  // When user pushes a button, program will get time from input box, 
  // increment (offset=1) or decrement (offset=-1) to next (or previous)
  // quarter hour and out value back to input box.

  // This is used on handheld devices where keyboard is very small and input is     
  // difficult and time consuming.

  var pass_time = document.getElementById(time_id).value;
  var tempDate;

  // break in to hours and minutes 
  var HH = pass_time.substr(0,2);
  var MM = pass_time.substr(2,4);
  // dummy date
  try{
  tempDate = new Date("2000", "01", "01", HH, MM, "00", "0000");
  }
  catch(err)
  {
   alert("invalid time (HHMM)");
   return;
  }

  // dummy minutes
  var minutes = 999;

  // iterate until we have reached an inter
  while (minutes != 0 && minutes != 15 &&  minutes != 30 && minutes != 45){
   tempDate.setMinutes( tempDate.getMinutes() + offset );
   minutes = tempDate.getMinutes();   
   document.getElementById(time_id).value = cleanUp(tempDate.getHours()) + "" + cleanUp(tempDate.getMinutes());
  }

}

function cleanUp(d)
{
  if (d < 10){
   d = "0" + d;
  }
  return d;
}

Answer 1

The while loop is very inneficient for this. I'd use setTimeout instead - call it every second if you need or every X increment you need.

Answer 2

Yeah, you can just calculate the distance to the next interval, then run the contents of the while loop on that value. You don't need to be constantly modifying the contents of the page with interim values for 'minutes'.

E.G., cast minutes to a float, divide by 15, use either the floor / ceil function for decrement / increment, then multiply by 15 again. That's just off the top of my head, it may not work for corner cases and such.