To make a valid portability election, the Internal Revenue Code (“Code”) requires an executor to make the election on an estate tax return filed within the “time prescribed by law” (including extensions) for filing that return.
The Code further requires estate tax returns be filed within nine months of the date of the decedent’s death. However, the Code also only, as a general rule, requires an estate tax return to be filed when the decedent’s gross estate exceeds his/her unused applicable exclusion amount (i.e., $5.25 million for 2013).
Estate Tax Portability – What Is A Timely Filed Estate Tax Return? – Forbes