Sitemap.xml: lastmod timestamp can contain invalid dates

[hostep]

Preconditions

1. Magento CE 2.1.5

Steps to reproduce

1. Have a category in the database where the created_at timestamp is a valid date, and the updated_at timestamp is 0000-00-00 00:00:00
   I don't think you can manage this by manually creating a category in the backend of Magento, but we managed to have some categories like this by importing them using a custom PHP script.
2. Generate a sitemap.xml file
3. Look at the generated sitemap.xml file for that category.

Expected result

1. The lastmod value in the xml should contain the created_at timestamp

Actual result

1. The lastmod value in the xml looks like this:

<lastmod>-001-11-30T00:00:00+00:00</lastmod>

The negative value is most likely caused by our timezone setting: Europe/Brussels.

Possible solution

diff --git a/app/code/Magento/Sitemap/Model/Sitemap.php b/app/code/Magento/Sitemap/Model/Sitemap.php
index 3b6f646f9a5..434393ae63a 100644
--- a/app/code/Magento/Sitemap/Model/Sitemap.php
+++ b/app/code/Magento/Sitemap/Model/Sitemap.php
@@ -341,9 +341,10 @@ class Sitemap extends \Magento\Framework\Model\AbstractModel
             $changefreq = $sitemapItem->getChangefreq();
             $priority = $sitemapItem->getPriority();
             foreach ($sitemapItem->getCollection() as $item) {
+                $lastmod = $item->getUpdatedAt() === '0000-00-00 00:00:00' ? $item->getCreatedAt() : $item->getUpdatedAt();
                 $xml = $this->_getSitemapRow(
                     $item->getUrl(),
-                    $item->getUpdatedAt(),
+                    $lastmod,
                     $changefreq,
                     $priority,
                     $item->getImages()

Discussion

I'm not sure if this is an actual bug in Magento, due to the fact that we inserted some data in the database in a possibly incorrect way.

Also: what should happen when the created_at timestamp contains '0000-00-00 00:00:00'? In that case the lastmod field shouldn't appear I assume, since it is optional: https://www.sitemaps.org/protocol.html#xmlTagDefinitions

If somebody could let me know what the best solution would be in this case, I can create a pull request for this.
If it is decided that this isn't an actual bug, then we should fix our custom import code I guess.

Please let me know, thanks! :)

[korostii]

IMHO setting updated_at to a meaningful value during the import seems most logical, regardless of the outcome of this issue.

Back to the issue in question, a negative date value is something new 🤣

[magento-engcom-team]

@hostep, thank you for your report.
The issue is already fixed in develop branch
But we will consider to backport the fix to patch releases

[hostep]

@magento-engcom-team: can you point me to the commit which fixes this, since I don't see it?
The last change to the last <lastmod> in the Magento\Sitemap\Model\Sitemap class was done 4 years ago ...

I think you are talking about issue 10598 which is not the same as this one :)