Sitemap index

<lastmod>

When a child sitemap last changed — the element that lets a crawler skip a file it has already seen.

An index <lastmod> describes the sitemap file, not the pages listed in it. Kept accurate, it is the cheapest optimisation in the whole protocol: a crawler comparing it against its last visit can skip an entire 50,000-URL file in one request.

The format is the same W3C Datetime as everywhere else, and the same discipline applies. An index whose children all carry today's date tells a crawler to refetch everything, every time.

Format: Sitemap index
Namespace: Sitemap protocol 0.9
Parent: <sitemap>
How many: At most one per <sitemap>.

What it contains

Nothing — this element holds a value rather than other elements. Its content is text, and the constraints below are what that text has to be.

Cardinality and constraints

At most one per <sitemap>.

W3C Datetime: a date, or a date and time with an offset.
Describes the child sitemap file, not the pages it lists.
Should change only when that child's contents change.

What our validator does

Coverage is reported as a warning: the validator says how many child sitemaps carry a lastmod and how many do not.

Broken and fixed

Both documents below are well-formed XML — that is the point of this corpus — and both are run through the validator on every test run. The verdicts here are what it returned, not what we expect it to.

broken

<?xml version="1.0" encoding="UTF-8"?>
<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
  <sitemap>
    <loc>https://example.com/sitemap-1.xml</loc>
  </sitemap>
</sitemapindex>

1 child sitemap(s) have no <lastmod>.

fixed

<?xml version="1.0" encoding="UTF-8"?>
<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
  <sitemap>
    <loc>https://example.com/sitemap-1.xml</loc>
    <lastmod>2026-08-10</lastmod>
  </sitemap>
</sitemapindex>

No errors. Warnings may remain — a conforming feed can still be missing a self link — but nothing here is wrong.

Where it reaches

Related elements

All 34 elements

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