Returns the duration between the starting and ending date.
function getDuration(startDate, endDate) {
  
  if(!(startDate instanceof Simple && endDate instanceof Simple)){
    throw new Error('Start and end dates must be simple dates');
  }
  
  var start = getObjFromDate(startDate, true),
      end = getObjFromDate(endDate, true),
      hasTime = false,
      duration = '';

  zipDates(start, end);

  if(end.seconds != undefined) {
    while(end.seconds-start.seconds < 0) {
      end.minutes -= 1;
      end.seconds += 60;
    }
    if(end.seconds-start.seconds > 0) {
      hasTime = true;
      duration = ('00'+(end.seconds-start.seconds)).substr(-2,2)+'S'+duration;
    }
  }

  if(end.minutes != undefined) {
    while(end.minutes-start.minutes < 0) {
      end.hours -= 1;
      end.minutes += 60;
    }
    if(end.minutes-start.minutes > 0) {
      hasTime = true;
      duration = ('00'+(end.minutes-start.minutes)).substr(-2,2)+'M'+duration;
    }
  }

  if(end.hours != undefined) {
    while(end.hours-start.hours < 0) {
      end.day -= 1;
      end.hours += 24;
    }
    if(end.hours-start.hours > 0) {
      hasTime = true;
      duration = ('00'+(end.hours-start.hours)).substr(-2,2)+'H'+duration;
    }
  }

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

  if(end.day != undefined) {
    while(end.day-start.day < 0) {
      end.day += GlobalUtil.daysInMonth(end.month,end.year);
      end.month -= 1;
      if(end.month < 1) {
        end.year -= 1;
        end.month += 12;
      }
    }
    if(end.day-start.day > 0) {
      duration = ('00'+(end.day-start.day)).substr(-2,2)+'D'+duration;
    }
  }

  if(end.month != undefined) {
    while(end.month-start.month < 0) {
      end.year -= 1;
      end.month += 12;
    }
    if(end.month-start.month > 0) {
      duration = ('00'+(end.month-start.month)).substr(-2,2)+'M'+duration;
    }
  }

  if(end.year-start.year > 0) {
    duration = ('0000'+(end.year-start.year)).substr(-4,4)+'Y'+duration;
  }

  if(end.year-start.year < 0 || duration == '') {
    throw new Error('Start Date must be less than End Date');
  }

  return new Duration('P'+duration);
}