public function getUnits()
    {
        return [
            self::UNIT_NONE   => 'None',
            self::UNIT_MINUTE => 'Minutes',
            self::UNIT_HOUR   => 'Hours',
            self::UNIT_DAY    => 'Days',
            self::UNIT_WEEK   => 'Weeks',
            self::UNIT_MONTH  => 'Months',
            self::UNIT_YEAR   => 'Years',
        ];
    }