Invalid Date Error in Google Webmaster for Sitemap generated by WordPress SEO by Yoast

If you have recently tested your sitemap that is generated by Yoast plugin for WordPress SEO, then you might have encountered an error in Google Webmaster tools while adding or testing the sitemap.

The error type would usually be mentioned as "invalid date". To fix this, one of the plugin file in Yoast SEO plugin need to be edited. Thanks to a contributor (roaming_elephant) in WordPress support forum, part of the code needs replacement. Here are the step-by-step procedures to fix this problem:

Step 1: open the plugin folder and open this php file: wordpress-seo/inc/class-sitemaps.php

Step 2: Go to line 249 and 250 where you will find the below code:

if ( $query->have_posts() )
 $date = $query->posts[0]->post_modified_gmt;

Replace the above code with the below given code:

if ( $query->have_posts() ) {
  $date = $query->posts[0]->post_modified_gmt;
  $date = date( 'c', strtotime( $date ));
}

That is all you need to do. You can now go to google webmaster tools and retest your sitemap that is generated by yoast. It should return no errors. If you have any questions, feel free to comment here.