Justin:
Yeah, It's probably splitting hairs. It seemed to me Clark was just about in the act of the stroke on his tee shot when the horn began to sound or was sounding. I wasn't paying all that close attention but my first thought was that the horn might bother him in his swing. Apparently not---he hit it very close and chose to mark and finish the hole out tomorrow morning.
BTW, as Lanny said, if one competitor gets his shot in before the horn blows (finishes blowing) the other competitor(s) have the option of teeing off after the horn and finishing the hole if they so choose. Did Clark's fellow competitor, Els, tee off after Clark? Also, competitors do not have to agree to all finish or none finish. In Els' case, obviously he could've chosen to tee off or not.
It seems like what you're really asking or want to ask is if Clark should be DQed for getting his swing in either during or just after the horn (or all the blasts were sounded). When committees suspend play for a dangerous situation or darkness the procedure is someone counts down over the radio and a number of officials are positioned around the course who blow their airhorns on the count down of 5,4,3,2,1. Not all the horns are completely simultaneous. Believe me, Justin, I hope there's no tournament committee out there who would even bother to check the tape on something as petty as Clark teeing off during or just after the horns sounded.
Generally speaking the idea of officiating is to help players and not look for some technical way to penalize them. I'm not saying any Rules official should overlook a real breach of a Rule but something like the suspension horn for darkness tonight in the Masters and what Clark did is not worth even looking into as a breach of the "Discontinuance" Rule.