Does Google penalize daily updated <lastmod> tags in sitemaps if the data is not daily updated?

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I've got a sitemap that is generated daily with a lot of links to product pages. These products are imported daily from another data source. Because the update consists of throwing away all current product info and replacing it with the new imported info the last modified date always jumps one day. This is also used in the sitemap. Even for products that haven't changed. All product pages pretend to have been updated.

Will Google penalize the website for pretending the pages have changed from day to day while they haven't?

My solution would be to only change the entry only if the new imported product data differs from the previous data. I just want to make sure this is a useful upgrade to make, while I could also spend my time on other improvements.

Solution :

I've never heard anything about a penalty due to this. At worst you're wasting the spider's time, but that's part of why we have computers in the first place: doing tedious repetitive things. Still, you should ideally be addressing the issue.

This...

My solution would be to only change the entry only if the new imported product data differs from the previous data.

...is what you should be doing in the first place, regardless of external considerations like sitemaps, etc. If your content isn't different(and I would include deleting and replacing with identical information in that description), then your lastmod date shouldn't be modified. Here you're wasting your own resources. You haven't said how many products are involved, but there's going to be a point where this process is going to get slow and computationally expensive.

I've never liked the idea of updating <lastmod> every day as itt's not just wrong, it's misleading search engines.

In a post over on SO, Google's Gary Illyes wrote:

The lastmod tag is optional in sitmaps and in most of the cases it's ignored by search engines, because webmasters are doing a horrible job keeping it accurate.

I've generally advocated for either using <lastmod> correctly, or not at all. Leaving it off (as well as <changefreq> & <priority>) even makes the file itself smaller and quicker for search engines to read as well.

No. Google will use lastmod as a hint (same as all sitemap values) but if it decides that your content is not getting updated daily then it will simply ignore it and revisit your pages on its own schedule.

I don't work for Google, and can't say for sure what they actually do, but the sensible way for them to treat <lastmod> timestamps would be as hints not to waste time re-crawling pages that haven't changed.

If the issue about seo, google-search, sitemap, is resolved, there’s a good chance that your content will get indexed and you’ll start to show up in Google search results. This means a greater chance to drive organic search traffic to your site.