code: public static Expression millisToStr(Expression expression, String format) {
          if (format == null || format.isEmpty()) {
              return x("MILLIS_TO_STR(" + expression.toString() + ")");
          }
          return x("MILLIS_TO_STR(" + expression.toString() + ", \"" + format + "\")");
      }
docs: Returned expression results in the string in the supported format to which
  the UNIX milliseconds has been converted.