Use Sitemaps Ping, Last Modified and Separate Sitemaps to Index Updated Content
To help Google index updated content more quickly, ping Googlebot when a Sitemap has been updated, use Last Modified dates in Sitemaps, and use a separate Sitemap for updated content so it can be crawled more frequently.
Last Modification Dates Important For Recrawling Changed Pages on Large Sites
Including last modification dates on large sites can be important for Google because it helps prioritize the crawling of a changed page which might otherwise take much longer to be recrawled.
Content Freshness Isn’t Always Used as a Ranking Signal
Content freshness isn’t always used as a ranking signal, as there isn’t always a need for pages to be updated regularly if they are still relevant. For example, academic research generally doesn’t change even though if it was written many years ago.
Provide Last Modified Date in Content & Structured Data For Rich Snippet
Providing the last modified date within the page’s content and as structured data makes it easier for Google to show this as a rich snippet in the SERPS, especially if both dates match.
Google Sometimes Uses ETags or Last-modified Headers to Reduce Bandwidth of Requests
ETags or last-modified headers can help Google to reduce the amount of bandwidth needed for requests, but these aren’t always used.
Use Sitemaps With Last Modified for Expired Content
Use a last modified date with a regularly updated Sitemap to help get expired pages picked up more quickly.
Identical Last Modified Dates in Sitemaps for all Pages will be ignored
If all the last modified dates in Sitemaps are identical, Google will assume they are incorrect.
Use Last Modified in Sitemaps for Updated Content
The best way to get pages re-crawled which have updated data is to submit in a Sitemap with the last modified date.
Google Measures Sitemap Trust
Google has a trust rating per Sitemap, based on incorrect use of last modified data. Google can learn that the last modified date is providing useful information which increases the trust. Otherwise it will start to ignore the last modified dates.