Why do airplanes take longer to fly West than East?

http://curious.astro.cornell.edu/about-us/40-our-solar-system/the-earth/climate-and-weather/68-why-do-airplanes-take-longer-to-fly-west-than-east-intermediate

The reason for the difference is an atmospheric phenomena known as the jet stream. ... The planes moving at a constant air speed thus go faster in the West-East direction when they are moving with the wind than in the opposite direction.