Takes in a start duration and a multiplier,
and returns a new Duration.
Rounds using Math.round
function multiplyDuration(startDuration, multiplier) {

  if(!isFinite(multiplier) || multiplier <= 0) {
    throw new Error('Invalid Multiplier');
  }

  var newDuration = {},
      hasTime = false,
      duration = '';

  if(startDuration.getSeconds()) {
    newDuration.seconds = Math.round(startDuration.getSeconds()*multiplier);
  }

  if(startDuration.getMinutes()) {
    newDuration.minutes = Math.round(startDuration.getMinutes()*multiplier);
  }

  if(startDuration.getHours()) {
    newDuration.hours = Math.round(startDuration.getHours()*multiplier);
  }

  if(startDuration.getDays()) {
    newDuration.days = Math.round(startDuration.getDays()*multiplier);
  }

  if(startDuration.getMonths()) {
    newDuration.months = Math.round(startDuration.getMonths()*multiplier);
  }

  if(startDuration.getYears()) {
    newDuration.years = Math.round(startDuration.getYears()*multiplier);
  }

  if(newDuration.seconds) {
    hasTime = true;
    duration = newDuration.seconds+'S'+duration;
  }

  if(newDuration.minutes) {
    hasTime = true;
    duration = newDuration.minutes+'M'+duration;
  }

  if(newDuration.hours) {
    hasTime = true;
    duration = newDuration.hours+'H'+duration;
  }

  if(hasTime) {
    duration = 'T'+duration;
  }

  if(newDuration.days) {
    duration = newDuration.days+'D'+duration;
  }

  if(newDuration.months) {
    duration = newDuration.months+'M'+duration;
  }

  if(newDuration.years) {
    duration = newDuration.years+'Y'+duration;
  }

  if(!duration) {
    throw new Error('Invalid Duration Multiplier');
  }

  return new Duration('P'+duration);

}