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You may not simply used any date up to 6 months from the actual date of death.
Your basis in property you inherit from a decedent is generally - the FMV of the property at the date of the individual's death.
The FMV on the alternate valuation date may be used if the personal representative for the estate chooses to use alternate valuation. For information on the alternate valuation date, see the instructions for Form 706 page 7 - http://www.irs.gov/pub/irs-pdf/i706.pdf
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