start date for incorporation

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).