In the default wordpress rss feed the element <pubDate> is

<pubDate><?php echo mysql2date( 'D, d M Y H:i:s +0000', get_post_time( 'Y-m-d H:i:s', true ), false ); ?></pubDate>

and gives a result of <pubDate>Tue, 12 May 2020 12:39:03 +0000</pubDate>

how can i change this line to give a result of <pubDate>12/05/2020 12:39:03 +0000</pubDate> (d/m/Y) or <pubDate>12-05-2020 12:39:03 +0000</pubDate> (d-m-Y) but to be valid with RFC-822 ?

I tried many variations <pubDate><?php echo mysql2date('r', get_the_time('Y-m-d H:i:s')); ?></pubDate> or <pubDate><?php echo get_post_time( 'd/m/Y H:i:s O', true ); ?></pubDate> but they dont validate.

Any ideas would be appreciated.