I agree with everyone who says the clock on the wall is what matters here.
That said, if you get in the situation where you haven't seen the light/heard the horn, but the clock says either 0.0 or 0:00, one way to confirm the light/horn hasn't malfunctioned is to check the box at the table if it has a digital display. I had a 0.0 situation once, and sure enough the box said 0.02. So we had another throw-in. And then we heard an exceptionally quick horn.
