Understanding the time scale of Madden–Julian oscillation (MJO) is the key to predict global weather patterns beyond two weeks. However, the time scale have remained mysterious as MJO theories vastly disagree on what assumptions are needed to explain the time scale. This study considers the pressure associated with alternating easterly and westerly winds around the equator and finds an intrinsic oscillation period of 36 to 64 days like the MJO given the long, narrow, and shallow flow geometry. All these geometric assumptions are well justified. This result suggests that the time scale of the MJO arises from its length scale.