code: public static Date addDays(long dt, int days)
      {
          Calendar c = getCalendar();
          if(dt > 0L)
              c.setTimeInMillis(dt);
          c.add(Calendar.DATE, days);
          return c.getTime();
      }
docs: Returns the given date adding the given number of days.
  @param dt The date to add the days to
  @param days The number of days to add. To subtract days, use a negative value.
  @return The date with the given days added