User blog comment:Dereviewer/Dipper's True Love/@comment-27159736-20150922163924/@comment-27159736-20150924181957

I said this another forum I'll explain in a bit more detail

Time Baby is worried about about Bill so since Time Baby is a omnipotent being he has probably traced the Moment Bill opens the portal and brings his friends to start the apocalypse.

Time Baby isolates Gravity Falls from the rest of the world and plans to destroy it so that Bill never arrives in Gravity Falls. it may impact the timeline but then again he can create time wishes so he may have the power to destroy the town without altering the timeline in a major way.

Mabel and Dipper have to stop Time Baby before he destroys the town but they don't understand his intentions because Time Baby is as vague as the other characters when it comes to the grand plan but at the end Bill arrives and brings the party to them or at least finds his new host to start the party.

However this unlikely to happen because it would be too much to do in one episode