Today is the filing deadline for 2016 income tax returns for calendar-year S corporations (Form 1120S) and partnerships (Form 1065). Apply for an automatic six-month filing extension using IRS Form 7004. Today is also the deadline for C corporations that want to have S corporation status for 2017 to make their election if they were in business in 2016. Be sure to check whether a separate state election for S status is required.