code: public static String dateToString(Date date) {
          GregorianCalendar c = new GregorianCalendar(GMT);
          c.setTime(date);
          String s = calendarToString(c);
          assert s.endsWith("+00:00");
          return s.substring(0, s.length() - 6) + "-00:00";
      }
docs: Return a String suitable for use as an edn {@code #inst}, given
  a {@link Date}.
  @param date must not be null.
  @return an RFC3339 compatible string.