MySQL add 12 hours to a time field

Try using ADDTIME instead of DATE_ADD. You could do SET start_time = ADDTIME(start_time, '12:00:00')

UPDATE `events` 
SET start_time = start_time + INTERVAL 12 HOUR
WHERE `start_time` < '11:00:00'

The MySQL functions that accept INTERVAL arguments are mostly unnecessary; you can just add and subtract intervals with + and -.

set start_time = ADDTIME(start_time,'12:00:00')

DATE_ADD works fine with timestamp etc, but not with TIME