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SEO Madmanby Adam Hafez
Free tool

Sitemap lastmod checker: W3C Datetime format audit

Sitemap lastmod checker: audit every lastmod in an XML sitemap against W3C Datetime and flag future and identical dates. Free, runs in your browser.

Runs entirely in your browser. Nothing you enter is uploaded, logged or stored.

Checked against the W3C Datetime forms allowed by the sitemaps.org protocol. Nothing you type is sent anywhere.

Key takeaways

  • The sitemaps.org protocol requires lastmod in W3C Datetime format, and YYYY-MM-DD is allowed when the time portion is omitted.
  • A timestamp needs a time zone designator, so 2026-09-12T10:00:00 without Z or an offset is reported as invalid.
  • The protocol says lastmod is the date the page was last modified, not the date the sitemap was generated.
  • Google says it uses lastmod only if it is consistently and verifiably accurate, so identical dates across every URL are flagged.
  • This checker tests format and plausibility only and cannot compare a date with the real last change of the page.

What it takes

  • A pasted XML sitemap

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How to use it

  1. 1Paste the sitemap XMLCopy the contents of one sitemap file, with its url entries, into the box. A sitemap index file has no page lastmod to audit here.
  2. 2Read the summary rowIt counts entries, invalid values and future dates so you can see at a glance whether the file needs work.
  3. 3Fix invalid formatsRewrite each flagged value as YYYY-MM-DD or a full timestamp with Z or an offset such as +00:00.
  4. 4Question identical datesIf every URL shares one date, feed lastmod from each page's real modification time, or omit it, rather than stamping the build time.

Paste one XML sitemap, and this lastmod checker tests every lastmod in it against the date formats the sitemaps.org protocol allows, then looks for values that are well-formed but cannot be right. It runs in your browser, and nothing you paste is sent anywhere.

What this sitemap lastmod checker tests

The sitemaps.org protocol says lastmod should be in W3C Datetime format and that YYYY-MM-DD may be used when the time is left out. The checker accepts these forms:

  • A year: 2026
  • A year and month: 2026-09
  • A full date: 2026-09-25
  • A date and time with a time zone: 2026-09-25T08:30Z, 2026-09-25T08:30:00+00:00, or with fractional seconds

It then checks each value in turn:

  • Format. Anything else, such as 09/12/2026 or a timestamp with no Z or offset, is a problem.
  • Real calendar date. A value in the right shape that does not exist, such as 2026-02-30 or an hour of 25, is a problem.
  • Future date. A value more than a day ahead is a warning. The protocol defines lastmod as the date the page was last modified, which cannot be in the future.
  • Identical values. If three or more entries all carry one date, that is a warning. Google says it uses lastmod only if it is consistently and verifiably accurate.
  • Missing values. Entries with no lastmod are counted as information, not an error, because the protocol makes the tag optional.
  • File size. More than 50,000 URL entries is a problem; the protocol and Google both cap one sitemap at 50,000 URLs.

How to read the results

The first card is the summary: entries found, how many are invalid and how many are in the future. It turns red if any value is invalid and amber if any is in the future. Each entry then gets its own card with its loc, its lastmod and a label: Pass, Warning, Problem or Info, with a one-line reason.

If the paste has no url elements at all, the checker says so. That usually means a sitemap index file, which lists sitemaps rather than pages; paste one of the child sitemaps instead.

Worked example

The prefilled sitemap has four URLs. The summary reads “4 entries, 2 invalid, 0 in the future”:

  • /: 2026-09-01 passes as a valid W3C Datetime.
  • /guide: 2026-09-10T08:30:00+00:00 passes.
  • /old-format: 09/12/2026 is a problem. It is not one of the W3C Datetime forms, and the order of day and month is ambiguous anyway.
  • /no-zone: 2026-09-12T10:00:00 is a problem. A time needs a time zone designator such as Z or +00:00.

Change all four values to one date and the identical-values warning appears above them.

Common mistakes it catches

  • Locale date formats, such as 09/12/2026, written by a CMS or spreadsheet export.
  • Timestamps with no time zone, often produced by a language’s default date formatting.
  • The build time stamped on every URL, which the protocol explicitly rules out: lastmod is when the page changed, not when the sitemap was generated.
  • Dates in the future, from a publish schedule or a clock set wrong.

What it cannot do is compare a date with the page. A valid, plausible lastmod can still be wrong, and Google says it should reflect the last significant update, so a changed copyright year does not count. Feed the value from the content’s real modification time, or leave it out.

Sources

  1. 1.Sitemaps XML format - sitemaps.orgPrimary
  2. 2.Build and submit a sitemap - Google Search CentralPrimary

Frequently asked questions

What date format should a sitemap lastmod use?

W3C Datetime, as the sitemaps.org protocol requires. The simplest valid form is YYYY-MM-DD, such as 2026-09-25. A full timestamp such as 2026-09-25T08:30:00+00:00 is also valid, as long as it ends with Z or a time zone offset.

Is lastmod required in a sitemap?

No. The protocol lists it as optional. Leaving it out is better than filling it with a date that is not the real modification date of the page.

Does Google use lastmod, changefreq and priority?

Google states it ignores priority and changefreq. It uses lastmod only when the value is consistently and verifiably accurate, and says it should reflect the last significant update to the page, which a changed copyright date is not.

Is a date without a time valid?

Yes. The protocol allows YYYY-MM-DD when the time portion is omitted. If a time is given, a time zone designator must follow it.

Why does the checker flag identical dates?

When three or more URLs all share one lastmod, the value is often the time the sitemap was built. The protocol says lastmod must be the date the page was modified, not when the sitemap was generated, so a flat date is a sign it cannot be trusted.

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