How far in advance would we know exactly _where_ on the Earth it will hit? If, say, New York is going to be destroyed, it makes a big difference if we know that a year in advance vs. a day.
Knowing where, will depend on the angle of impact - if came basically straight in (tangent to the surface) we can be pretty accurate, but the more shallow the angle the harder it will be.
With a shallow angle, we'll probably be able to tell the latitude pretty well, but not the longitude.
Also, if you evacuate New York a year in advance, who's to say that the prediction isn't just a skosh off so that New York is left untouched but the real impact location was hit unprepared?