What date do I use for the "acquired date" for stock options exercised before the IPO?

I exercised stock options prior to leaving a company over 10 years ago.  Their stock only went public in 2013.  I didn't sell my shares until 2016.  What date do I put for the date I acquired the stock? The date I exercised the options and purchased shares in the company? or the date of the IPO when the shares were converted into stock?