What is a Last-Modified header?
The date and time the resource was last changed, used for cache validation via If-Modified-Since.
Last-Modified gives a simple, human-readable timestamp for when a website's resource last changed, and it pairs with If-Modified-Since to let a client skip re-downloading content that hasn't been updated since its last visit. It's a coarser, timestamp-based alternative to ETag's content fingerprint, and the two are commonly used together for more robust cache validation.
Common use case
A static file server reports the file's actual filesystem modification time as Last-Modified, letting browsers and caches skip re-downloading it on a later visit if it hasn't changed.
Example
Last-Modified: Sat, 22 Aug 2026 09:15:00 GMTHistory
Part of the original HTTP/1.0 specification, RFC 1945 (1996).
Did you know?
Because it only has second-level precision, a resource that changes more than once within the same second can go undetected by clients relying on Last-Modified alone - one reason ETag is often paired with it.