Special thank you to RachieVee for getting the ball rolling with their comment.
I figured out how to do this, so I thought I might as well document it for future reference by others. On a side note, this only pertains to the _S theme, or any theme based off of that starting template.
Alright, so it takes about 5 minutes to fix this “double date” – after checking the content.php file, I found <?php my_underscores_theme_name_posted_on(); ?> where the meta output was for the date and time information. After doing a little digging, I ran into the my_underscores_theme_name.pot file -and on lines 257 to 260, the following information was written:
#: inc/template-tags.php:82
msgctxt "post date"
msgid "Posted on %s"
msgstr ""
I deduced that this was what I needed, yet it was still too limiting. So I went into the php file referenced (inc/template-tags.php) at line 82, and changed it to look like so (for your case, replace league_of_legends_rift with whatever your theme name is.):
if ( ! function_exists( 'league_of_legends_rift_posted_on' ) ) :
/**
* Prints HTML with meta information for the current post-date/time and author.
*/
function league_of_legends_rift_posted_on() {
$time_string = '<time class="entry-date published" datetime="%1$s">%2$s</time>';
$time_string = $time_string;
if ( get_the_time( 'U' ) !== get_the_modified_time( 'U' ) ) {
$time_string2 = ', last updated on ';
$time_string2 .= '<time class="updated" datetime="%1$s">%2$s</time>';
}
$time_string = sprintf( $time_string,
esc_attr( get_the_date( 'c' ) ),
esc_html( get_the_date() )
);
$time_string2 = sprintf( $time_string2,
esc_attr( get_the_modified_date( 'c' ) ),
esc_html( get_the_modified_date() )
);
$posted_on = sprintf(
_x( 'Posted on %s', 'post date', 'league-of-legends-rift' ),
'<a href="' . esc_url( get_permalink() ) . '" rel="bookmark">' . $time_string . '</a>'.$time_string2
);
$byline = sprintf(
_x( 'by %s', 'post author', 'league-of-legends-rift' ),
'<span class="author vcard"><a class="url fn n" href="' . esc_url( get_author_posts_url( get_the_author_meta( 'ID' ) ) ) . '">' . esc_html( get_the_author() ) . '</a></span>'
);
echo '<span class="posted-on">' . $posted_on . '</span><span class="byline"> ' . $byline . '</span>';
}
endif;
This it looked slightly different before, a lot more condensed with one variable ($time_string) which would have your original publish date and your update date appended to each other. I used the logic already present and introduced a second variable, $time_string2 which I used to store the phrase and information for the updated date. Then I modified the $posted_on to include this new variable and the formatting that I wanted – mind you, to edit this appended output you’ll now have to do it from this php file, rather then the “.pot” file I referenced earlier.
Hope this helped you as much as it helped me!