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For a decedent's estate, the moment of death determines the end of the decedent's tax year and the beginning of the estate's tax year. As executor or administrator, you choose the estate's tax period when you file its first income tax return. The estate's first tax year may be any period of 12 months or less that ends on the last day of a month. If you select the last day of any month other than December, you are adopting a fiscal tax year.
To change the accounting period of an estate, use Form 1128, Application To Adopt, Change, or Retain a Tax Year - www.irs.gov/pub/irs-pdf/i1128.pdf
According to instructions - To request a ruling to adopt, change, or retain a tax year, file Form 1128 by the due date (not including extensions) of the federal income tax return for the first effective year.
If Form 1128 was not timely filed - there still are options for late election.
Generally, an application filed after the appropriate due date stated above is considered late. However, applications filed within 90 days after the due date may be considered as timely filed under Regulations section XXX.XXXX-X when the applicant establishes that:
The taxpayer acted reasonably and in good faith and
Granting relief will not prejudice the interests of the government.
Applications that are filed more than 90 days after the due date are presumed to jeopardize the interests of the Government, and will be approved only in unusual and compelling circumstances.
Under either circumstance, an extension request must be filed under Procedure and Administration Regulations section XXX.XXXX-X and is a ruling request under Rev. Proc. 2011-7, 2011-1 I.R.B. 233 at www.irs.gov/irb/2011-01_IRB/index.html and is subject to public inspection under section 6110. See section 7 of Rev. Proc. 2011-1, 2011-1 I.R.B. 1 at www.irs.gov/irb/2011-01_IRB/index.html (updated annually) for information on requesting a ruling.