code: public static Duration getRelativeDaylightSavingsOffset(Date self, Date other) {
          Duration d1 = getDaylightSavingsOffset(self);
          Duration d2 = getDaylightSavingsOffset(other);
          return new TimeDuration(0, 0, 0, (int) (d2.toMilliseconds() - d1.toMilliseconds()));
      }
docs: Return a Duration representing the DST difference (if any) between two
  dates.  i.e. if one date is before the DST changeover, and the other
  date is after, the resulting duration will represent the DST offset.
  
  @param self  a Date
  @param other another Date
  @return a Duration