My site has "Tegalog" embedded, so I'm putting it to good use.

In function.php:

function get_tegalog_last_updated() {

    $rss_content = @file_get_contents('※Directory');

    if (!$rss_content) return date('Y年n月j日'); // ★Returns today's date

    $rss_content = mb_convert_encoding($rss_content, 'UTF-8', 'auto');

    $rss = simplexml_load_string($rss_content);

    $latest_item = $rss->channel->item[0] ?? null;

    if ($latest_item && isset($latest_item->pubDate)) {

        return date('Y年n月j日', strtotime((string)$latest_item->pubDate));

    }

    return date('Y年n月j日'); // ★Returns today's date

}

index.php contains

<div class="last-updated">

    <p>Last Updated: <?php echo get_tegalog_last_updated(); ?></p>

</div>

Since this was CGI, it was easy to handle,
but for Instagram, a plugin might be easier.