Which one is correct?
I will be at the office at 9.
Or
I will be in the office at 9
It depends on the context. Stating that you will be at the office at 9 implies that you shall arrive at 9. However, stating that you will be in the office at 9 implies that you shall continue to be in the office at 9.
They are both correct. This ngram shows that "in the office" is about twice as popular as "at the office".