public Object format(long timestamp) {
        return dateTimeFormatter == null ? timestamp : dateTimeFormatter.format(Instant.ofEpochMilli(timestamp));
    }
@Test
    void testFormatTimestampAsString() {
        TimestampFormatter timestampFormatter = new TimestampFormatter("yyyy-MM-dd'T'HH:mm:ss.SSSZ",
            ZoneId.of("GMT+01:00"));
        assertThat(timestampFormatter.format(timestamp)).isEqualTo("2017-12-22T16:30:31.000+0100");
    }