public static TimeUnit timestampPrecisionStringToTimeUnit(String timestampPrecision) {
    if ("us".equals(timestampPrecision) || "microsecond".equals(timestampPrecision)) {
      return TimeUnit.MICROSECONDS;
    } else if ("ns".equals(timestampPrecision) || "nanosecond".equals(timestampPrecision)) {
      return TimeUnit.NANOSECONDS;
    } else {
      return TimeUnit.MILLISECONDS;
    }
  }
convert duration string to millisecond, microsecond or nanosecond.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/utils/DateTimeUtils.java