- September 15 – Third quarter installment of 2016 individual estimated income tax is due.
- September 15 – Filing deadline for 2015 tax returns for calendar-year corporations that received an automatic extension of the March filing deadline.
- September 15 – Filing deadline for 2015 tax returns for partnerships that received an extension of the April filing deadline.
- October 1 – Generally, the deadline for businesses to adopt a SIMPLE retirement plan for 2016.
- October 17 – Deadline for filing 2015 individual tax returns on extension.
- October 17 – Deadline for reconverting a Roth IRA to a regular IRA.
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