code: public static String toFtsUtcString(Date date) {
          if (date == null) {
              return null;
          }
          DateFormat rfc3339 = df.get();
          String zDate = rfc3339.format(date);
          String xDate = zDate.replaceFirst("\\+0000$", "Z");
          return xDate;
      }
docs: Converts a date to the default string representation in FTS (RFC 3339).
  
  @param date the {@link Date} to convert.
  @return the RFC 3339 representation of the date, in UTC timezone (eg. "2016-01-19T14:44:01Z")