lastmod is an optional tag in a sitemap. It records the date, and sometimes the time, that a page was last changed. Search engines use it to decide which pages are worth fetching again. Most content management systems and SEO plugins fill it in automatically.
<url>
<loc>https://example.com/pricing</loc>
<lastmod>2026-10-08T00:00:00.000Z</lastmod>
</url>Why it matters here
Reading every page of a large site on every check would be slow, and most pages have not changed. If a page’s date has not moved since the last check, there is usually nothing new on it, so monitoring skips it and spends its effort on pages that did change. How to tell whether your dates are reliable is in check your sitemap dates.
How monitoring uses it
| Page | Read again when |
|---|---|
| New, or its date changed | On the next check. |
| Unchanged, with a date | In a rotating slice of about 5% of the site (capped at 2,000 pages), and in a full pass about every 30 days. |
| No date, site of up to 20,000 pages | On every check. |
| No date, larger site | About every 7 days up to 200,000 pages, and every 14 days beyond that. |
The rotating slice and the full pass exist because a date can be wrong, or a link can be added without the page’s date moving. Dates also help a sampled scan: when a sitemap is bigger than a plan reads, the most recently changed pages are part of the sample.
Common misunderstandings
- Wrong dates make monitoring slower or later, not wrong. If every date changes on every request, everything is read every time. If dates never change, a changed page waits for the slice or the full pass.
- A hacked page often keeps its old date, because injected code does not update the CMS. That is why unchanged pages are still sampled.
- A one-off scan does not skip anything for being unchanged. Dates only matter to monitoring and to choosing a sample.
Questions
Does my sitemap need lastmod?
No. Without it, monitoring still works, but it reads pages on the fixed schedule in the table above rather than skipping the unchanged ones.
Why does everything look changed?
Some plugins write the current date into every entry. Look at your sitemap in a browser and compare the dates with when you last edited the pages.
Add a site to monitoring and see it at work.
