ISO8601 date should be ATOM

Isn't ATOM a subset of ISO8601? How is this a bug? Why am I not surprised to see your username in this issue. ^^

I should have worded my issue better.
When I read, “An ISO8601 formatted date”, I am assuming: DATE_ISO8601. According to the PHP documentation, this is: DATE_ISO8601 (example: 2005-08-15T15:52:01+0000).
A DATE_ATOM (example: 2005-08-15T15:52:01+00:00) is exactly 1 character longer.
Also, an example “‘lastmod’ => ‘2012-10-12T17:40:30+02:00’,)” requires first looking up which date characters should be used, while “‘lastmod’ => date(DATE_ATOM, $changed)” can be copied directly into program code.

When a developer decides to use DATE_ISO8601, he will see that DATE_ISO8601 is deprecated and DATE_ATOM should be used instead. However, the <lastmod> date can be formatted in different ways and ATOM format is not the only option.