Adds a duration to a date, returning the new date.
function addDuration(startDate, duration) {
  var end = getObjFromDate(startDate, false),
      endString = '';

  // Initialize all the values we need in end based on the duration
  zipDuration(end, duration);

  // Add Timezone offset to endString
  if(startDate.getTZHours() != undefined) {
    if(startDate.getTZHours() < 0) {
      endString += '-';
    } else {
      endString += '+';
    }
    endString += ('00'+Math.abs(startDate.getTZHours())).substr(-2,2);
    endString += ':'+('00'+Math.abs(startDate.getTZMinutes())).substr(-2,2);
  }

  if(duration.getSeconds()) {
    end.seconds += duration.getSeconds();
  }
  while(end.seconds && end.seconds >= 60) {
    end.seconds -= 60;
    end.minutes += 1;
  }
  if(end.seconds != undefined) {
    endString = ':'+('00'+end.seconds).substr(-2,2)+endString;
  }

  if(duration.getMinutes()) {
    end.minutes += duration.getMinutes();
  }
  while(end.minutes && end.minutes >= 60) {
    end.minutes -= 60;
    end.hours += 1;
  }
  if(end.minutes != undefined) {
    endString = ':'+('00'+end.minutes).substr(-2,2)+endString;
  }

  if(duration.getHours()) {
    end.hours += duration.getHours();
  }
  while(end.hours && end.hours >= 24) {
    end.hours -= 24;
    end.day += 1;
  }
  if(end.hours != undefined) {
    endString = 'T'+('00'+end.hours).substr(-2,2)+endString;
  }

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

  if(duration.getMonths()) {
    end.month += duration.getMonths();
  }
  while(end.month && end.month > 12) {
    end.month -= 12;
    end.year += 1;
  }
  if(end.month != undefined) {
    endString = '-'+('00'+end.month).substr(-2,2)+endString;
  }

  if(duration.getYears()) {
    end.year += duration.getYears();
  }
  if(end.year != undefined) {
    endString = ('0000'+Math.abs(end.year)).substr(-4,4)+endString;
    if(end.year < 0) {
      endString = '-'+endString;
    } else {
      endString = '+'+endString;
    }
  }

  // After adding year we could have bumped into a non leap year
  // TODO fix this

  if(end.year > 9999) {
    throw new Error('New date out of range');
  }

  // TODO return actual simple or approximate dates
  if(startDate.isApproximate()) {
    endString = 'A'+endString;
    return new Approximate(endString);
  } else {
    return new Simple(endString);
  }

}