code: function cloneDateByFlag(d, clone) {
      if (_utc(d) && !isDefined(optFromUTC)) {
        optFromUTC = true;
      }
      if (_utc(d) && !isDefined(optSetUTC)) {
        optSetUTC = true;
      }
      if (clone) {
        d = new Date(d.getTime());
      }
      return d;
    }
docs: Force the UTC flags to be true if the source date date is UTC, as they will be overwritten later.