Here is the
advice from the TurboTax help content for when the date acquired is blank on
your Form 1099-B:
When it comes to the date acquired, your 1099-B can still be
useful (even though it left this area blank). As long as each sale was listed
as either short-term or long-term, you can type in the word "Various"
in the box for date acquired.
Please refer to the link below for the information on the stock's cost basis.
https://ttlc.intuit.com/replies/3300454