Problem with the last modified date

How does Yoast determine the last modified date for categories? Store-category is a taxonomy I use, it’s a category of a custom post type called “store”. The date is wrong, its format is strange, see here:
http://www.dizengof-center.co.il/store-category-sitemap.xml

The date is also wrong for the sales sitemap, although the items inside this sitemap have the right date:
http://www.dizengof-center.co.il/sitemap_index.xml

https://wordpress.org/plugins/wordpress-seo/