_buildValue(firstDateTimestamp, lastDateTimestamp) {
      this._prevDelimiter = this._effectiveDelimiter;
      if (firstDateTimestamp) {
        const firstDateString = this._getStringFromTimestamp(firstDateTimestamp * 1000);
        if (!lastDateTimestamp) {
          return firstDateString;
        }
        const lastDateString = this._getStringFromTimestamp(lastDateTimestamp * 1000);
        return `${firstDateString} ${this._effectiveDelimiter} ${lastDateString}`;
      }
      return "";
    }
Builds a string value out of two UTC timestamps - this method is the counterpart to _extractFirstTimestamp/_extractLastTimestamp
@private