If a spouse dies and his/her estate does not fully use the decedent’s remaining unified credit, the surviving spouse can use the unused credit if certain conditions are met. One of the conditions is that an estate tax return is filed for the decedent that makes a portability election, even if no estate tax return is otherwise needed.
An estate tax return is due 9 months after death, with an automatic extension granted for 6 months if timely requested. When no return is required, a return being made solely to make a portability election can be filed late, up to two years after death of the first spouse, if the extension procedures of Rev.Proc. 2017-34 are followed (generally, filing a return within that time period while adding a specified legend to the top of the return).
This two year period has now been extended to 5 years. This was principally due to the IRS receiving too many Form 9100 requests (requiring a private letter ruling) after the two year period but often within 5 years. To save IRS resources, the extension period is now 5 years.
Note that if it turns out that an estate tax return was otherwise required, the extension is null and void.