Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Harrisburg to Bismarck as you normally would.
There are currently 3+ open flights from Harrisburg to Bismarck within the next 7 days for less than $700.
Great question - MDT to BIS just means Harrisburg Airport to Bismarck Airport. Now you’ll know what these unique codes when you look for flights!