static long guessTimestamp(Span span) {
    if (span.timestampAsLong() != 0L) return span.timestampAsLong();
    for (Annotation annotation : span.annotations()) {
      if (0L < annotation.timestamp()) {
        return annotation.timestamp();
      }
    }
    return 0L; // return a timestamp that won't match a query
  }
gets the timestamp in milliseconds floored to midnight
zipkin-tests/src/main/java/zipkin2/storage/ITDependencies.java