What is holding date for Mylan stockholders since the merger with Viatris?

In many stock-swap mergers, you keep your existing holding period.

According to this article from Zacks.com (their emphasis):

If you trade old shares for new through a merger or acquisition, the IRS does not look on the event as a taxable transaction. It doesn't matter whether the shares are preferred, common or private; nor does it matter whether the trade was voluntary on your part or if you voted for it. Your original investment has not been disposed of, as far as tax liability is concerned, and no capital gain or loss has to be reported.

TheStreet.com says something similar (emphasis mine):

If the deal is a tax-free stock swap, the acquiring company will swap your shares for shares in its company. This generally is not a taxable event, and you keep the original basis and holding period, says Almon. So if you bought your original shares at $10, that would be your basis in the shares of the acquiring company after the swap. You will not have to worry about capital gains tax until you decide to sell those new shares.

However, according to this Form 8937 from Viatris (emphasis mine):

The receipt of Viatris common stock in exchange for Mylan ordinary shares pursuant to the Combination was a taxable transaction for U.S. federal income tax purposes. Therefore, generally a Mylan shareholder recognized capital gain or loss equal to the difference between (i) the shareholder's adjusted tax basis in the ordinary shares of Mylan stock exchanged and (ii) the fair market value of the shares of Viatris common stock received in the transaction. A shareholder's adjusted tax basis in Mylan ordinary shares generally should have equaled the shareholder's purchase price for the shares, as adjusted to take into account stock dividends, stock splits, or similar transactions.

A Mylan shareholder receiving Viatris common stock pursuant to the transaction will have an initial tax basis in those Viatris common stock shares equal to the fair market value of the Mylan ordinary shares exchanged therefor.

So Viatris structured this merger such that you will need to pay capital gains tax on your Mylan stock as if you sold it on the day of the merger. And now you have a fresh holding of Viatris stock with a new start date and a new basis.