Display Author Registration Date

How about reformat it instead of stripping?

//Get post author ID
$post_author_id = get_post_field( 'post_author', $post->ID );

//Get the registration date
$registered_date = get_the_author_meta( 'user_registered', $post_author_id );

//Convert to desired format
$output = date( 'F j, Y', strtotime($registered_date));

//Echo
echo $output;