Stock bought in May 2018. Dec 2018 the company was acquired & my stock changed to the new company. I sold the stock in Oct 2019. May 2018 should be the date acquired ?

The date you acquired the stock will be May of 2018. The change in company ownership does not affect the original purchase date.

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?