Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Surabaya to Da Nang as you normally would.
Related info for your journey
Useful info, stats and facts about Surabaya to Da Nang flights.
These are just IATA (International Air Transport Association) codes. They are used in aviation to identify and differentiate airports. That’s why a Surabaya Juanda Airport to Da Nang Airport flight would often be abbreviated as a SUB to DAD flight.