Check your records and use the date from your records for this exact transaction. How did you pay for the purchase? Can you check your bank records or whatever method you would have used to purchase the assets? Any reasonable method is generally acceptable if you are using the best of your ability.
If you are planning to use an estimate, you must first have records that show whether it was a long-term or short-term holding period for supporting documentation in case IRS questions. We are not saying to just make up a date. There has to be some reasonable method used.
If you nor the brokerage company have no records that show the dates or whether it was long-term or short-term, choosing to use the short-term holding period would be the safer choice, but less advantageous on your return than the long-term holding period when dealing with IRS.
