calculate datetime and time use PHP-Mysql

mysql> SET @mulai2 = '1985-03-01 12:00:00', @elapse2 = '12:00:00';

mysql> SELECT TIMESTAMP(`mulai2`,`elapse2`) from tbl_backupmfb2

+---------------------------------+

| TIMESTAMP(`mulai2`,`elapse2`)   |

+---------------------------------+

| 1986-03-02 00:00:00             |

+---------------------------------+