Fuzzy cites a Canadian source. For US taxpayers, it's the trade date unless a short sale is involved. This is from IRS 2017 Instructions for Form 8949: "Use the trade date for stocks and bonds traded on an exchange or over-the-counter market. For a short sale, enter the date you delivered the property to the broker or lender to close the short sale. If you received a Form 1099-B (or substitute statement), box 1c may help you determine when you sold or disposed of the property."
