It depends on the design of the clock. At least two styles exist....most, I think, operate like CountTheBasket is implying.

The clock, internally, starts at 30.000000 but only displays 30.

At the very instant it is turned on, it internally drops to 29.999999. However, it may either still show 30 (rounding up) or it may show 29 (rounding down). In either case, it will remain the same until is crosses below 29.000000.

In the case of a clock that rounds up for the display, the violation would occur the moment it shows 20. In the case of a clock that rounds down for the display, the violation would occur when it shows 19.

An easy way to tell is to see if the horn sounds the instant it reaches 0 (a clock that rounds up) or if there is a delay of 1 second between the horn reaching 0 and the horn (a clock that rounds down).

I believe that most, if not all, modern clocks operate by rounding up so that when it shows 0, it is truly 0.0000000 and not 0 and a fraction.....or that when it shows any whole number, it has passed below that whole number but not yet reached the next whole number.

Only if he was using an NBA shot clock.

MOST game clocks will "instantly" shift from 20:00 to 19:59 -- the tenths are still there, they just aren't shown; the clock therefore "rounds down." IF the clock doesnt' display tenths, then there will be a second delay between the clock showing 0:00 and the horn.

MOST shot clocks stay at 30 (or 35) for a second before changing. They round up. THe horn sounds as soon as the clock shows 0.

In any event, the rule / mechanics are clear to blow the whistle as soon as the clock hits 20 (or 25).

As Bob said, for NCAAW the violation occurs when the shot-clock reads :20 or on the tenth second after the ball is legally touched inbounds, whichever applies. That's the interpretation we received from the moment the rule was established.

By rule (1-19-2 to be exact) "two visible shot clocks are required, and shall be recessed and mounted on the backboard supports behind each backboard." 1-19-3 calls for an "alternate timing device" to be available if the visible clock malfunctions but there's the rub: the visible clocks did work. They jumped but the horn went off at 30 seconds. The fact the alternate - and hard-wired - shot-clocks were having issues made it a tough decision. Our R figured we were better off with all four active. One of the alternates had to be shut down because it started jumping worse than those on the backboards. At any rate, both teams were willing to deal.