code: @Override
      public void serialize(
              final ZonedDateTime date,
              final JsonGenerator generator,
              final SerializerProvider provider) throws IOException, JsonProcessingException {
          generator.writeString(date != null ? DateUtils.toStringIsoFormat(date) : null);
      }
docs: Serializes the given <code>ZonedDateTime</code> in a format that is readable by commonly use
  libraries.
  
  @see <a href=
  "http://www.joda.org/joda-time/apidocs/org/joda/time/format/ISODateTimeFormat.html#dateOptionalTimeParser--">dateOptionalTimeParser</a>
  @see <a href=
  "https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-date-format.html#mapping-date-format">strict_date_optional_time</a>