Gary Illyes from Google said that you are better off not using a lastmod date in your XML Sitemap if those dates are wrong. He said on Bluesky, "probably better off without the lastmods. at least you save a few bytes."
This answer was in response to Jason Kilgore's question, which asked:
Would a site be better off with no lastmod date at all on any pages? Or one where the lastmod date isn't reliable/trusted (given the binary nature of Google's trust on this)? We found a bug manifesting a set of unintentionally incorrect dates.
This really isn't new advice, as Google has said before that Google would trust or not trust the lastmod date based on if that date has been used correctly on your site or not. You should only change the lastmod date for significant changes to the web page, only when there is something new, not for copyright changes, Google said. Google also spoke about changing the lastmode date too frequently, It was so bad that back in 2015, Google said it mostly ignored the lastmod date but in 2017 changed course with that language. John Mueller said changing the lastmod date is not an SEO hack.
Here are those new posts on this topic that I covered above:
In short, use the lastmod date correctly and don't try to trick Google.
Forum discussion at Bluesky.
