TO_DAYS(date) returns the number of days since the first day in the year 1 A.D. (there was no year 0).
The opposite effect is obtained by using FROM_DAYS(number).
mysql>
mysql> select TO_DAYS('2000-01-01') ;
+-----------------------+
| TO_DAYS('2000-01-01') |
+-----------------------+
| 730485 |
+-----------------------+
1 row in set (0.00 sec)
mysql>
mysql> SELECT FROM_DAYS(1000000);
+--------------------+
| FROM_DAYS(1000000) |
+--------------------+
| 2737-11-28 |
+--------------------+
1 row in set (0.02 sec)
mysql>