Verification of the method to obtain merger time using the circularity-age distribution. Shown are probability density distributions in phase space of stellar age, t, versus circularity, λ_z, for TNG50 468590, TNG50 375073, TNG50 220596, and TNG50 2, from Panel (a) to Panel (d). In each panel, the left plot is the true distribution for the stellar orbits within 2 R_e directly obtained from the simulation, and the vertical dashed line marks the merger end time of the last massive merger the galaxy has experienced. The merger end time is consistent with the time t|_{fdisk = 0.5} in TNG50 468590, TNG50 375073, and TNG50 220596, with still significant star formation after the last massive merger, and it is consistent with the end of star formation in TNG50 2. The right plot is the distribution from our best fitting model, and the vertical dashed line marks the time t|_{fdisk = 0.5} in the model for TNG50 468590, TNG50 375073, and TNG50 220596, while it marks the end of star formation in TNG50 2.