Here is an article that has some commentary and addresses your question:
https://finance.zacks.com/determine-shares-sell-fifo-lifo-9766.html
So in your example, it is the January purchase. Also keep in mind that since this is a ST gain, ordinary income rates will apply.
*A reminder that posts in a forum such as this do not constitute tax advice.
Also keep in mind the date of replies, as tax law changes.
