in ecs-logging-core/src/main/java/co/elastic/logging/TimestampSerializer.java [112:118]
private CachedDate(long epochTimestamp) {
SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd", Locale.ROOT);
dateFormat.setTimeZone(TimeZone.getTimeZone("UTC"));
cachedDateIso = dateFormat.format(new Date(epochTimestamp));
startOfCachedDate = atStartOfDay(epochTimestamp);
endOfCachedDate = atEndOfDay(epochTimestamp);
}