How do I complete the blanc "date of acquisition", when there is no date in my form 1099-B?

[MichaelL1]

You will have to estimate the best you can as to when the asset was purchased.

If it was purchased more than one year earlier than when sold is the big difference as sales of assets held more than one year get Long Term Gain treatment which is more favorable.

So for example if I sold something I think I bought 10 years ago, but not sure of date I would just pick a date about 10 years ago and document in my tax file what I did and why. IRS only cares that it was more than a year ago.