static long compressDateTime(DateTime time, int dayStep) {
        if (dayStep > 1) {
            int days = Days.daysBetween(DAY_ONE, time).getDays();
            int groupBucketOffset = days % dayStep;
            return Long.parseLong(time.minusDays(groupBucketOffset).toString(TIME_BUCKET_FORMATTER));
        } else {
            /**
             * No calculation required. dayStep is for lower traffic. For normally configuration, there is pointless to calculate.
             */
            return Long.parseLong(time.toString(TIME_BUCKET_FORMATTER));
        }
    }
Follow the dayStep to re-format the time bucket literal long value.

Such as, in dayStep == 11,

20000105 re-formatted time bucket is 20000101, 20000115 re-formatted time bucket is 20000112, 20000123
re-formatted time bucket is 20000123
oap-server/server-storage-plugin/storage-elasticsearch-plugin/src/main/java/org/apache/skywalking/oap/server/storage/plugin/elasticsearch/base/TimeSeriesUtils.java