The get_feed_build_date() WordPress PHP function retrieves the UTC time of the most recently modified post from WP_Query.
Usage
echo get_feed_build_date('D, d M Y H:i:s T');
Parameters
$format(string) – The date format string to return the time in.
More information
See WordPress Developer Resources: get_feed_build_date()
Examples
Display last modified post date in RSS feed
In this example, we display the last modified post date in the RSS feed using the get_feed_build_date() function.
header('Content-Type: ' . feed_content_type('rss-http') . '; charset=' . get_option('blog_charset'), true);
$more = 1;
echo '<?xml version="1.0" encoding="' . get_option('blog_charset') . '"?' . '>';
?>
<rss version="2.0"
xmlns:content="http://purl.org/rss/1.0/modules/content/"
xmlns:wfw="http://wellformedweb.org/CommentAPI/"
xmlns:dc="http://purl.org/dc/elements/1.1/"
xmlns:atom="http://www.w3.org/2005/Atom"
xmlns:sy="http://purl.org/rss/1.0/modules/syndication/"
xmlns:slash="http://purl.org/rss/1.0/modules/slash/"
>
<channel>
<title><?php bloginfo_rss('name'); ?></title>
<atom:link href="<?php self_link(); ?>" rel="self" type="application/rss+xml" />
<link><?php bloginfo_rss('url') ?></link>
<description><?php bloginfo_rss('description') ?></description>
<lastBuildDate><?php echo get_feed_build_date('D, d M Y H:i:s T'); ?></lastBuildDate>
<language><?php bloginfo_rss('language'); ?></language>
<sy:updatePeriod><?php echo apply_filters('rss_update_period', 'hourly'); ?></sy:updatePeriod>
<sy:updateFrequency><?php echo apply_filters('rss_update_frequency', '1'); ?></sy:updateFrequency>
<?php do_action('rss2_head'); ?>
<?php while (have_posts()) : the_post(); ?>
<item>
<title><?php the_title_rss() ?></title>
<link><?php the_permalink_rss() ?></link>
<pubDate><?php echo mysql2date('D, d M Y H:i:s +0000', get_post_time('Y-m-d H:i:s', true), false); ?></pubDate>
<dc:creator><![CDATA[<?php the_author() ?>]]></dc:creator>
<guid isPermaLink="false"><?php the_guid(); ?></guid>
<description><![CDATA[<?php the_excerpt_rss(); ?>]]></description>
<content:encoded><![CDATA[<?php the_content_feed('rss2'); ?>]]></content:encoded>
<wfw:commentRss><?php echo esc_url(get_post_comments_feed_link(null, 'rss2')); ?></wfw:commentRss>
<slash:comments><?php echo get_comments_number(); ?></slash:comments>
<?php rss_enclosure(); ?>
<?php do_action('rss2_item'); ?>
</item>
<?php endwhile; ?>
</channel>
</rss>