public function toUserDate($mTimestamp = null, $sFormat = null)
    {
        $oConverted = static::convert($mTimestamp, $this->sTimezoneUser, $this->sTimezoneNails);

        if (is_null($oConverted)) {
            return null;
        }

        if (is_null($sFormat)) {
            $sFormat = $this->sUserFormatDate;
        }

        return $oConverted->format($sFormat);
    }