@User Here v shows data falling, 0.037 is the setup time of the DFF_X1, which is also the violating slack, as your input external delay matches the clock period. Time is not decreasing, there are just 2 final calculations: clock riding edge - setup time = data required time 10.000 - 0.037 = 9.963 data required time - data arrival time = slack 9.963 - 10.000 = -0.037 OpenSTA reports are not special and very similar to others (Xilinx, Synopsys, etc.), so you'll find some docu\video on the internets.