The date you acquired the stock will be May of 2018. The change in company ownership does not affect the original purchase date.
**Mark the post that answers your question by clicking on "Mark as Best Answer"
The correct answer to your question depends entirely on if the acquisition was a taxable or non-taxable acquisition. If it was a taxable acquisition then I'd expect you'd have received a 1099-B for 2018 in which case the new stock would have a Dec 2018 acquisition date. Did you get a 2018 1099-B associated with the acquisition? Names of companies involved?