Forwardport of #11902 to 2.3-develop branch

Author: serhii-balko

app/code/Magento/Sitemap/Model/Sitemap.php

@@ -42,6 +42,11 @@ class Sitemap extends \Magento\Framework\Model\AbstractModel implements \Magento
 
     const TYPE_URL = 'url';
 
+    /**
+     * Last mode date min value
+     */
+    const LAST_MOD_MIN_VAL = '0000-01-01 00:00:00';
+
     /**
      * Real file path
      *
@@ -177,6 +182,13 @@ class Sitemap extends \Magento\Framework\Model\AbstractModel implements \Magento
      */
     private $sitemapItemFactory;
 
+    /**
+     * Last mode min timestamp value
+     *
+     * @var int
+     */
+    private $lastModMinTsVal;
+
     /**
      * Initialize dependencies.
      *
@@ -694,7 +706,11 @@ protected function _getMediaUrl($url)
      */
     protected function _getFormattedLastmodDate($date)
     {
-        return date('c', strtotime($date));
+        if ($this->lastModMinTsVal === null) {
+            $this->lastModMinTsVal = strtotime(self::LAST_MOD_MIN_VAL);
+        }
+        $timestamp = max(strtotime($date), $this->lastModMinTsVal);
+        return date('c', $timestamp);
     }
 
     /**

app/code/Magento/Sitemap/Test/Unit/Model/SitemapTest.php

@@ -527,7 +527,7 @@ protected function getModelMock($mockBeforeSave = false)
             ->willReturn([
                 new SitemapItem('category.html', '1.0', 'daily', '2012-12-21 00:00:00'),
                 new SitemapItem('/category/sub-category.html', '1.0', 'daily', '2012-12-21 00:00:00'),
-                new SitemapItem('product.html', '0.5', 'monthly', '2012-12-21 00:00:00'),
+                new SitemapItem('product.html', '0.5', 'monthly', '0000-00-00 00:00:00'),
                 new SitemapItem(
                     'product2.html',
                     '0.5',

app/code/Magento/Sitemap/Test/Unit/Model/_files/sitemap-1-3.xml

@@ -10,7 +10,7 @@
         xmlns:image="http://www.google.com/schemas/sitemap-image/1.1">
     <url>
         <loc>http://store.com/product.html</loc>
-        <lastmod>2012-12-21T00:00:00-08:00</lastmod>
+        <lastmod>0000-01-01T00:00:00-08:00</lastmod>
         <changefreq>monthly</changefreq>
         <priority>0.5</priority>
     </url>

app/code/Magento/Sitemap/Test/Unit/Model/_files/sitemap-single.xml

@@ -22,7 +22,7 @@
     </url>
     <url>
         <loc>http://store.com/product.html</loc>
-        <lastmod>2012-12-21T00:00:00-08:00</lastmod>
+        <lastmod>0000-01-01T00:00:00-08:00</lastmod>
         <changefreq>monthly</changefreq>
         <priority>0.5</priority>
     </url>