public static long midnightUTC(long epochMillis) {
    Calendar day = Calendar.getInstance(UTC);
    day.setTimeInMillis(epochMillis);
    day.set(Calendar.MILLISECOND, 0);
    day.set(Calendar.SECOND, 0);
    day.set(Calendar.MINUTE, 0);
    day.set(Calendar.HOUR_OF_DAY, 0);
    return day.getTimeInMillis();
  }
For bucketed data floored to the day. For example, dependency links.
zipkin/src/test/java/zipkin2/TestObjects.java