Before revision: Consider a panel data setting with observations of N units over T time steps. Each of the N units undergoes exactly one of D interventions at time step T_0, with 1 \le T_0 < T, prior to which all units experience no intervention, i.e., control.
After revision: Consider a panel data setting with observations of N units over T time periods. Each unit undergoes one of D interventions at time step T_0, with 1 \le T_0 < T, prior to which all units experience no intervention, i.e., control.