start date for incorporation
If a company incorporated in February 2009, is it proper to consider all transactions, even those occurring in January in the same set of books?
If a company incorporated in February 2009, is it proper to consider all transactions, even those occurring in January in the same set of books?

Likely not. If the incorporation changed the type of return being filed, definitely not. So an entity taxed as a sole proprietorship, then incorporated as an entity taxed as something else, would need two sets of books: one for the sole proprietorship, and one for the something else.
If, however, you had an LLC-taxed-as-a-corporation, and in February did what's called a "formless conversion" under state law to a state-law corporation, that would be a situation where a new set of books wouldn't be needed (or appropriate).