Google's John Mueller explained that changing the dates in your XML Sitemaps lastmod date to all show today, the most recent day, won't favor your rankings or SEO. He said on Reddit, "setting today's date in a sitemap file isn't going to be something that works in favor of anyone, it's just lazy."
This is a super old topic, something we discussed here countless times, so what is one more story on it.
John's full response was that these types of SEO hacks, or whatever you want to call it, may have worked for a short while, but they do not have long term benefits. And with the lastmod date, that wouldn't work.
He wrote:
I don't know your situation, so it's impossible to say. There are certainly some sneaky things that "work" for a while (usually it's just a while), and it's frustrating to be in a situation with a competitor like that. However, setting today's date in a sitemap file isn't going to be something that works in favor of anyone, it's just lazy. In the early days, it was often a sign that the sitemap generator was confused or broken (if a page is dynamically generated, theoretically it's "always updated"). It's trivial for search engines to recognize, and only makes it harder for them to recognize updated pages. This definitely isn't working in their favor.
Google has been warning that you should only change the lastmod date for significant content changes. Not for things like changing your copyright date in the footer or other less significant changes. But SEOs or maybe their CMS platforms are just too trigger happy with lastmod dates. In fact, Google may end up not trusting it.
So don't try to fool Google with your lastmod date, it should not work.
Forum discussion at Reddit.
