Off by ≤ 0.010s: godlike · ≤ 0.030s: pass · > 0.030s: fail. Your score is your win streak.
Perfect Second has exactly one rule: stop the clock at 1.000 seconds. The numbers vanish after 0.3s, so you're counting blind from there, and being off by a single millisecond breaks your streak. It's free, it's simple to explain, and almost nobody's good at it.