I am sure this addressed somewhere but I can't find it anywhere.

I purchased a stock in July 2010, but the company was acquired by another company in August 2013. It was an all stock deal (0.3 shares of new company). I sold the stock in September 2013.

Is my acquisition date for capital gains purposes July 2010 (long term) when I bought the original stock, or August 2013 (short term) when I acquired the stock of the new company?

Thank you.