What is lastmod in a sitemap?

lastmod is an optional XML tag inside each <url> entry that tells crawlers when the page last changed. The value must be in W3C date format — either YYYY-MM-DD or a full ISO 8601 timestamp (2024-05-12T14:30:00Z). When you convert a sitemap to CSV, lastmod becomes a separate column. It's the most useful sitemap field for analytics: sort by lastmod to find stale content, filter for recent changes to plan re-crawls, or compare two snapshots over time to detect publish patterns. Google uses lastmod to prioritize re-crawling, but ignores it entirely if you set it to the current date on every URL — that abuse pattern is detected and the field gets discounted.