Part of the fun of these things is proving that you started at midnight or later on The Day, but it certainly isn't required. The honor system is what's really in effect here. Frankly, I wouldn't personally care if people started building now, what's the diff? The point is to build the model.
That said, as this is a Mad Build, it seems to be a bit odd to start early. It's kind of like going down and secretly opening your presents before Christmas, then sealing them up again. You may be the only one who knows you did it, but it's still only you who suffers from it, because the fun of a Mad Build is the doing of it, the accomplishment.
If I remember, I will take a picture of the untouched sprues near the TV with the crawl of the TV station that has the date and time on it. But I didn't remember to do it for Mad Fokker, but that didn't stop me from starting on midnight!