LastModified

LastModified is a metadata timestamp indicating when an object was last changed. In computing, the term is most commonly associated with three contexts: the HTTP Last-Modified header, a file system modification time (mtime), and a general application programming attribute used to track updates to data objects.

In HTTP, the Last-Modified response header conveys the date and time the resource was last modified. The value uses an HTTP-date, typically in GMT, for example: Last-Modified: Tue, 15 Nov 2022 12:45:00 GMT. Clients can use the If-Modified-Since request header to ask for the resource only if it has changed, allowing servers to respond with 304 Not Modified when appropriate. This mechanism supports caching and reduces bandwidth. The header is defined in RFC 7232 and related standards; accuracy depends on the server's clock and the granularity of the underlying data.

In file systems, Last Modified corresponds to the modification time (mtime) stored in file metadata. It updates when file content changes and is used by backup, synchronization, and build tools to detect changes. Tools such as ls or stat display it.

In software and databases, LastModified can be a field representing the most recent update timestamp for an object or row. It is often used with other validation tokens, such as ETags, to detect concurrent modifications and support optimistic concurrency control.

Limitations include time zone handling, clock skew, and variable precision across systems. Some systems favor ETag or a combination of LastModified and size to validate cache entries.