I'm not entirely sure I understand your questions.  You can form a corporation at any time of the year, and you can take a salary from the corporation.  Income you earned prior to the formation of the corporation  and any salary you receive will be reported on your personal tax return.  The corporation's income will be reported on its tax return.  If you make an election under subchapter S, then the corporation's "income, losses, deductions, and credits [will be passed] through to [the corporation's] shareholders for federal tax purposes."