Nibiru can alter course at any moment, that's why TPTB have such a hard time tracking it and preparing to move to the bunkers.

Even the "Doctor" that spotted the dark object behind Elenin noted that it stops and then starts moving again.

Nibiru is of intelligent design and intelligent control, meaning that it can appear instantly without warning.

There really is no way to tell when it will make it's approach... Theoretically speaking, the final decision is up to the ruler of the planet, Lord Anu... If you believe that Nibiru is indeed inhabited by the Annunaki.

We are technically Anu's grandchildren, and we have grown a lot... There is probably a lot of discussion going on right now about what to do with our civilization. We have come so far that it seems like a shame to wipe us all out at once, but who knows what Anu has in store for humanity... There is really no way to tell.