It's often misattributed to Borges. It was written (in prose) by columnist Don Herold in 1953 for the _Reader's Digest_. Since then it underwent several changes, has been attributed to several authors, was broken into a verse form, was translated to Spanish, acquired a Spanish title, "Instantes", was then re-translated back into English often retaining the Spanish title...

It's lived a long and colorful life. But Borges didn't write it.

There's a detailed study of this poem's history at http://www.borges.pitt.edu/bsol/iainst.php. It's in Spanish, but Google Translate can give you the gist of it in English.