The IRS has released final regulations that set the length of the extension available for partnership and estate/trust tax returns at 5 months.  The regular due date of those returns is April 15 (for calendar-year taxpayers), so the extended due date is set at September 15.  This had been the case in practice since 2009 because of temporary regulations issued by the IRS, but these final regulations make it official.

The final regulations apply to businesses that file Form 1065 (partnerships and LLCs taxed as partnerships) and Form 1041 (estates or trusts).