Sitemap
A machine-readable list of the addresses a site wants found, with the date each was last changed.
It helps with discovery, not with ranking. Listing a page does not make it indexed and does not improve its position; what it does is shorten the road to pages that few links point at, which is why it matters most on large sites and on new ones.
The last-modified date is the part worth getting right, and the part most often wrong. A build that stamps every entry with the moment of deployment tells the crawler that the entire site changed, which is the same as telling it nothing — and it is exactly how a feed built from a sitemap starts reporting news that does not exist.
It is also read by more than search engines now. Sites without a feed are followed through their sitemap, so a correct date there is the difference between being noticed the day something is published and being noticed by accident.