_extractFirstTimestamp(value) {
      if (!value || !this._checkValueValidity(value)) {
        return undefined;
      }
      const dateStrings = this._splitValueByDelimiter(value); // at least one item guaranteed due to the checks above (non-empty and valid)
      const parsedDate = this.getFormat().parse(dateStrings[0], true);
      return parsedDate.getTime() / 1000;
    }
Returns a UTC timestamp, representing the first date in the value string or undefined if the value is empty
@private