Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Swansea to Dundee as you normally would.
Due to multiple factors, expect to potentially pay more when booking in July. For low rates, December can be a great choice for travel to Dundee. Flights to Dundee from Swansea in December are about £138 on average during December, but can be found for as low as £48.