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Measurements of the irreversibility line in Bi2Sr2CaCu2Oy single crystals are made by three experimental methods: by dc magnetization measurements, microwave absorption measurements, and registration of the harmonics in the high-frequency response of the superconductor. It is found that the position of the irreversibility line on the magnetic phase diagram depends substantially on the method of measurement and the frequency used. Analysis of the results shows that the discrepancy in the irreversibility lines is due to both a difference in the mechanisms of hysteresis of the magnetic properties and to the different scales of the measurement times. It is shown that the position of the irreversibility line is determined at low temperatures by the limiting value of the vortex creep rate for the given method of registration and at high temperatures by the time required for two-dimensional vortices to overcome the surface barrier. It is concluded that the irreversibility line is a dynamic phase transition. The first experimental evidence of the presence of a surface barrier for 2D vortices is obtained.