Black points: intervals Δt between successive bursts from FRB 121102, relative to the time t since the start of observations as recorded by Zhang et al. (2018). The source was active at the start; hence, Δt/t is artificially enhanced at small t. Because the distribution of Δt/t flattens at large t, we may estimate the time that the source had been active before the start of observations by adding an offset time to t. The gold points show the result for an offset of 0.1 hr.