Add DATE and TIME fields to get DATETIME field in MySQL

It should be as easy as

UPDATE table SET datetime_field = CONCAT(date_field, " ", time_field);

Both of the other answers do not convert the date properly if use use a TIME of "838:00:00" which is a valid time according to the mysql manual

so instead you can try converting the time field to seconds and then adding them
for example:

date_field + INTERVAL TIME_TO_SEC(time_field) SECOND

This will convert the date accordingly