public String toDescription() {
      String description =
          "Expects a time value with unit " +
          "(d/day, h/hour, m/min, s/sec, ms/msec, us/usec, ns/nsec)" +
          ", which is " + HiveConf.stringFor(timeUnit) + " if not specified";
      if (min != null && max != null) {
        description += ".\nThe time should be in between " +
            timeString(min) + (minInclusive ? " (inclusive)" : " (exclusive)") + " and " +
            timeString(max) + (maxInclusive ? " (inclusive)" : " (exclusive)");
      } else if (min != null) {
        description += ".\nThe time should be bigger than " +
            (minInclusive ? "or equal to " : "") + timeString(min);
      } else if (max != null) {
        description += ".\nThe time should be smaller than " +
            (maxInclusive ? "or equal to " : "") + timeString(max);
      }
      return description;
    }
validate value for a ConfVar, return non-null string for fail message
common/src/java/org/apache/hadoop/hive/conf/Validator.java