It's quite a good question, but I think the explanation of the answer could have been a bit better, since some may have reached the right answer for the wrong reason (or for insufficient reason) and need educating, and some will have got it wrong and want to know how one would go about getting it right (short of cheating and running the query).

The three ways to get it right are to do some pretty awful arithmetic, to cheat by running in in management studio, or eliminate obviously wrong answers until there's only one answer left. The first is not something that most people would want to contemplate, while the second is cheating. The last approach works just fine. However the explanation might usefully have made some of the points that working out the answer by that approach has to go through - for example that the range of the datetime type is 8247 years, which is more than 435000 days so the integer to be cast isn't out of range; then more people would be likely to learn from it.