public Object format(long timestamp) {
        return dateTimeFormatter == null ? timestamp : dateTimeFormatter.format(Instant.ofEpochMilli(timestamp));
    }
@Test
    void testDoNotFormatTimestamp() {
        TimestampFormatter timestampFormatter = new TimestampFormatter(null,
            ZoneId.of("GMT+01:00"));
        assertThat(timestampFormatter.format(timestamp)).isEqualTo(timestamp);
    }