Last

The Last-Modified response HTTP header contains a date and time when the origin server believes the resource was last modified. It is used as a validator to determine if the resource is the same as the previously stored one. Less accurate than an ETag header, it is a fallback mechanism. Conditional requests containing If-Modified-Since or If-Unmodified-Since headers make use of this field.

Last-Modified is also used by crawlers to adjust crawl frequency, by browsers in heuristic caching, and by content management systems (CMS) to display the time the content was last modified.