Feed updated property is invalid

According to the W3 Feed Validation Service, the "updated" property of the feed is not in compliance with RFC-3339. The four offset digits should have a colon in the middle, and at the moment, they don't.

For example, this feed is not valid:

<?xml version="1.0" encoding="utf-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">
  <title>Blog posts</title>
  <link rel="alternate" type="text/html" hreflang="en" href="http://example.com/" />
  <link rel="self" type="application/atom+xml" href="http://example.com/blog.atom" />
  <id>http://example.com/blog.atom</id>
  <updated>2020-04-13T20:37:41+0000</updated>
</feed>

Whereas this one is:

<?xml version="1.0" encoding="utf-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">
  <title>Blog posts</title>
  <link rel="alternate" type="text/html" hreflang="en" href="http://example.com/" />
  <link rel="self" type="application/atom+xml" href="http://example.com/blog.atom" />
  <id>http://example.com/blog.atom</id>
  <updated>2020-04-13T20:37:41+00:00</updated>
</feed>