Simple definition
The Last-Modified HTTP header indicates a resource modification time and can support conditional requests.
Purpose and use
Last-Modified reports the last-modification time assigned by the server to an HTTP representation.
How does it work?
A client can send it in If-Modified-Since for validation; a 304 response then permits reuse of an already stored representation.
Example and verification
If an update remains invisible, compare the reported timestamp with actual changes and other cache-freshness rules.
Points to watch
Second-level precision or poorly maintained timestamps can limit this validator. Last-Modified is neither a complete editing history nor a download timestamp.
In short
A reliable date helps revalidation, while a suitable ETag can distinguish versions that the timestamp alone cannot separate.