static long flooredTraceTimestamp(List<Span> trace) {
    long midnightOfTrace = Long.MAX_VALUE;
    for (Span span : trace) {
      long currentTs = guessTimestamp(span);
      if (currentTs != 0L && currentTs < midnightOfTrace) {
        midnightOfTrace = midnightUTC(currentTs / 1000);
      }
    }
    assertThat(midnightOfTrace).isNotEqualTo(Long.MAX_VALUE);
    return midnightOfTrace;
  }
gets the timestamp in milliseconds floored to midnight
zipkin-tests/src/main/java/zipkin2/storage/ITDependencies.java