How should I provide date acquired for a sold stock on 1099-B when I purchased stock many years ago, but have reinvested dividends, causing small monthly purchases?

I purchased a number of shares 10+ years ago.  The dividends were reinvested so small amounts were purchased every month.  In 2017 I sold some shares.  Should I use the original date of purchase, or some pro-rated amount, or just enter 'Various'?