Tracking the time evolution of tracer stars (i.e., those that would be observed by Gaia; orange) in the GC-1 simulation. After identifying these stars at the present time (top panel), we determine the time since the disruption of the stream as the epoch when at least half of all stars in the stream were contained within 5r_t (t = 1.83 Gyr; bottom left panel) or 2r_t (t = 1.53 Gyr; bottom right panel). The lookback time estimated from the z = 0 proper motions of the tracer stars (using Equation (5)) ﹩{t}^{l}{b}_{\mathrm{PM}}﹩ = 3.01 Gyr.