References:
IRS Instructions for Form 8949,
"Column (c)—Date Sold or Disposed Of
Enter in this column the date you sold or
disposed of the property.
For a short sale, enter the date you delivered
the property to the broker or lender to
close the short sale."
However, If you have a gain, see also IRS Pub 550 Chapter 4.
"Exception for constructive sales. Entering
into a short sale may cause you to be treated as
having made a constructive sale of property. In
that case, you will have to recognize gain on the
date of the constructive sale. For details, see
Constructive Sales of Appreciated Financial Positions,
earlier."
