public static String formatTimestampMillis(long millis) {
    return Instant.ofEpochMilli(millis).toString().replace("Z", "+00:00");
  }
@Test
  public void formatTimestampMillis() {
    String timestamp = "1970-01-01T00:00:00.001+00:00";
    assertThat(DateTimeUtil.formatTimestampMillis(1L)).isEqualTo(timestamp);
    assertThat(ZonedDateTime.parse(timestamp).toInstant().toEpochMilli()).isEqualTo(1L);

    timestamp = "1970-01-01T00:16:40+00:00";
    assertThat(DateTimeUtil.formatTimestampMillis(1000000L)).isEqualTo(timestamp);
    assertThat(ZonedDateTime.parse(timestamp).toInstant().toEpochMilli()).isEqualTo(1000000L);
  }