- Class
- C3 Standard
- Weight
- 2
- Severity on fail
- minor
- Applies to
- the Sitemaps protocol
Test this rule on a live site with the XML sitemap checker. Free, no account needed.
Scope and evidence
Evaluated on each sitemap file with lastmod values. The book marks this requirement STANDARD.
Fail conditions
Each condition has a stable code and a reason template. The evaluator fills the placeholders from observed values only; no sentence in a report is generated.
invalid_lastmod{n} lastmod values in {sitemap} are not in W3C Datetime format, for example “{example}”. The Sitemaps protocol specifies that format.
The documented fix
Serialize lastmod as an ISO 8601 date-time with a time zone, for example 2026-09-18T09:30:00+00:00.
Sources, quoted
These are the passages the rule rests on, stored once in the knowledge base and emitted exactly as written. A citation marked for specific conditions appears only on findings with those codes.
“This date should be in W3C Datetime format.”
[SITEMAPS-ORG] Sitemaps XML format (protocol) · XML tag definitions (lastmod) · last updated 2016-11-21
https://www.sitemaps.org/protocol.html