Bug in GetSitemapUrlForPost(): wrong URL formed

Hi,

I noticed from my logs that Googlebot was trying to fetch non-existent sub-sitemap URLs and went into the plugin’s source code.

I see that when pinging the post’s sitemap, the URL is formed from the post’s modification date, rather than the creation date (GetSitemapUrlForPost() in sitemap-builder.php).

If I created a page in April 2017 and then modified it in November, then the resulting sub-sitemap URL will have the suffix 2017-11 – but that sitemap is empty since I didn’t create any new posts/pages in November. The suffix should be based on the creation date, 2017-04.

Alternatively, posts could be sorted into sub-sitemaps by modification date, but that seems like a bad idea.

Googlebot doesn’t seem to be reacting to the fact that the modification date of the correct sub-sitemap in the sitemap index has changed – it only tries to fetch the non-existent sub-sitemap that was pinged.

I wonder if you could have a look at this? Your plugin is the best XML sitemap plugin out there.

Thank you!