Box 1b on the 1099-B is blank because the broker didn't have the information. But you can't leave it blank on your tax return. You have to provide the acquisition date from your own records of the purchase.
If you sold shares of stock or a mutual fund that you acquired through multiple purchases you can enter "Various" instead of a date, but you have to correctly classify it as short-term or long-term. If it is partially short-term and partially long-term, you have to split it up and report it as two separate sales.
If you inherited the investment that you sold, you can enter "Inherited" instead of a date. A sale of an inherited investment is always treated as long-term, no matter how long you or the person you inherited it from actually owned it.
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