When flying from Myrtle Beach to Plattsburgh, the time of day you decide to fly out makes little to no difference towards the overall price you’ll pay on tickets.
These are just IATA (International Air Transport Association) codes. They are used in aviation to identify and differentiate airports. That’s why a Myrtle Beach Intl Airport to Plattsburgh Intl Airport flight would often be abbreviated as a MYR to PBG flight.