IRS form 1099-B box 1c is date sold or disposed.
The financial institution did not report the date acquired because because it did not have that information. You may enter the trade date or the date when the trade took place if that information is available in your trading records.
The software uses the dates that you bought and sold the items to compute your holding period, long-term versus short-term. Does the IRS form 1099-B at least report a correct holding period?