The valid syntax of the query in MySQL?

Choose * FROM courses WHERE (now() + 2 hrs) > start_time

*note: start_time is really a area of courses table*

Choose *

FROM courses

WHERE DATE_ADD(NOW(), INTERVAL 2 HOUR) > start_time

You'll need DATE_SUB() OR DATE_ADD()

The DATE_ADD() function will have the desired effect. (You may also make use of the ADDTIME() function if you are running a minimum of v4.1.1.)

Choose * FROM courses WHERE (NOW() + INTERVAL 2 HOUR) > start_time