I have a client who sold stock this year but the 1099-B does not have a cost basis. However, on the 1099-B , there is a date shown in the date acquired column and the form also shows the # of shares. How would I calculate the cost basis... Do I look up the historical price for that stock on the date it was acquired and then multiple that price by the # of shares to calculate the price? Or option B, do I look up the historical price of the stock on the date the stock was inherited by the grandson, and do the calculation on that date?