First, you have to report a basis, even if it's zero. It is not uncommon for taxpayers to use a zero basis when they have no idea what the actual cost basis is. The IRS does not automatically treat a transaction as short term if you use zero as the basis. Even if you don't know the acquisition date, you can use "Various" and then indicate the transaction was long term or short term. As for incorrectly reporting the cost basis and transaction type, you are intentionally misrepresenting the facts on your tax return. When you sign your return (whether by hand to mail in or electronically to e-file) you are attesting to the following statement "Under penalties of perjury, I declare that I have examined this return and accompanying schedules and statements, and to the best of my knowledge and belief, they are true, correct, and complete." That said, if your cost basis and transaction type is reported to the IRS by your broker, and you misrepresent the facts, they won't match with what the IRS receives and most likely they will catch it and send you a letter letter letting you know they are changing your return due to an error on your tax return. If it causes you to lose a credit on your return, then you will owe additional tax, penalty, and interest.
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