Используйте to_days
:
select to_days(concat(year(now()),'-12-31')) - to_days(now()) as days_left;
+-----------+
| days_left |
+-----------+
| 121 |
+-----------+
или используйте дата как этот
select datediff(concat(year(now()),'-12-31'), now()) as days_left;
+-----------+
| days_left |
+-----------+
| 121 |
+-----------+