If you add to or subtract from a date value something that contains a time part, the result is automatically converted to a datetime value. : DATE_ADD : Date Time Functions MySQL TUTORIALS

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If you add to or subtract from a date value something that contains a time part, the result is automatically converted to a datetime value.


mysql>
mysql> SELECT DATE_ADD('1999-01-01', INTERVAL 1 DAY);
+----------------------------------------+
| DATE_ADD('1999-01-01', INTERVAL 1 DAY) |
+----------------------------------------+
| 1999-01-02                             |
+----------------------------------------+
1 row in set (0.00 sec)

mysql>