Why it matters
Google uses lastmod only if it is consistently accurate; lastModified: new Date() stamps every URL with the build time, so Google learns to ignore it.
How to fix it
Use each page's real content-modified date (from CMS or git) in W3C Datetime format.
References
More seo rules
void seo reports 60 rules in this category. What search engines need from the raw response: status codes, titles and descriptions in <head>, self-referencing canonicals, one h1, crawlable links, Open Graph, icons, robots.txt and a sitemap with real dates.
robots-sitemap-directiverobots.txt has no absolute Sitemap: linesitemap-missingNo reachable sitemapsitemap-invalidSitemap isn't valid sitemap XML or exceeds protocol limitssitemap-url-invalidSitemap <loc> is relative or on another hostsitemap-url-statusSitemap lists URLs that don't return 200 directlysitemap-url-not-indexableSitemap lists noindex, robots-blocked or non-canonical URLsduplicate-titleMultiple pages share the same <title>duplicate-descriptionMultiple pages share the same meta description