Ahh, but what to do if it is one of those clocks that rolls to 0 when 0.9 seconds remain? You see the clock show zero, the horn blows nearly a second after you see the time show 0? Hmmm.
I'm with BITS...if my whistle is clearly and distinctly before the horn, I am going to put time back on the clock...no ifs, ands, or buts about it.
Even if I can't see the clock, I have a good enough sense of time to have definite knowledge. You better come up with something....especially if the two (whistle/horn) are not almost simultaneous.