Their would imply that it mentions a human. However, in this case, it's describing a nonliving thing, or "tomorrow." Their should only be used in the case of humans, or a grouping of humans.
A. Complex sentence.
It contains an independent clause and one or more than one dependent clause.
An example is: After the game ended, he went back home.
Where "after the game ended" is the dependent clause and "he went back home" is the independent clause.
Hope this helps :)
