Crear DATETIME a partir de DATE y TIME

Copiado de la documentación de MySQL:

TIMESTAMP(expr), TIMESTAMP( expr1, expr2)

mysql> SELECT TIMESTAMP('2003-12-31');
    -> '2003-12-31 00:00:00'
mysql> SELECT TIMESTAMP('2003-12-31 12:00:00','12:00:00');
    -> '2004-01-01 00:00:00'