Flight Gimpo International Airport departs from Seoul Gimpo International Airport at 12:40 and arrives at Tokyo Haneda Airport at 14:55.
The flighttime is about 2h 15m. The distance travelled ("as the bird flies"): 1450 Km. So the average speed is roughly 644 Km.
The best day to arrive at Seoul is Saturday and it would be best to avoid Monday, as prices are on average 9.34% higher than on Saturday. (Average prices, based on 1082351 datapoints.)
We have a lot more statistics for flights NH 864 and between Seoul and Tokyo.