- September 17 – Third quarter installment of 2012 individual estimated income tax is due.
- September 17 – Filing deadline for 2011 tax returns for calendar-year corporations that received an automatic extension of the March 15 filing deadline.
- September 17 – Filing deadline for 2011 partnership tax returns that received an extension of the April 17 filing deadline.
- October 1 – Generally, the deadline for businesses to adopt a SIMPLE retirement plan for 2012.
- October 15 – Deadline for filing 2011 individual tax returns on extension.
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