- September 15 – Third quarter installment of 2015 individual estimated income tax is due.
- September 15 – Filing deadline for 2014 tax returns for calendar-year corporations that received an automatic extension of the March 16 filing deadline.
- September 15 – Filing deadline for 2014 partnership tax returns that received an extension of the April 15 filing deadline.
- October 1 – Generally, the deadline for businesses to adopt a SIMPLE retirement plan for 2015.
- October 15 – Deadline for filing 2014 individual tax returns on extension.
