Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Medan to Chiang Mai as you normally would.
These are just IATA (International Air Transport Association) codes. They are used in aviation to identify and differentiate airports. That’s why a Medan Kuala Namu Intl Airport to Chiang Mai Airport flight would often be abbreviated as a KNO to CNX flight.