Adding Seconds to datetime in MySQL

TIMESTAMPADD(SECOND,15,NOW())

unit = SECOND seconds to add = 15

UPDATE table end_dt = DATE_ADD(end_dt, INTERVAL 15 second)
WHERE DATE_SUB(end_dt, INTERVAL 15 second) <= NOW()

I think that's what you want, basically adds 15 seconds to end_dt when end_dt is 15 seconds away from now

EDIT NEW QUERY This query should work:

UPDATE `table`
    SET end_dt = DATE_ADD(end_dt, INTERVAL (15 - TIMESTAMPDIFF(SECOND, NOW(), end_dt)) SECOND)
WHERE DATE_SUB(end_dt, INTERVAL 15 second) <= NOW()