Last-Modified

Gives the time a selected representation was last changed.

Use Last-Modified with If-Modified-Since or If-Unmodified-Since when an ETag is unavailable or as a secondary validator.

httpheadercacheconditional

Reference (http)

Last-Modified: Wed, 21 Oct 2015 07:28:00 GMT

Use Last-Modified with If-Modified-Since or If-Unmodified-Since when an ETag is unavailable or as a secondary validator.

Common mistakes

Using a low-resolution timestamp as the only protection against concurrent writes.

IANA registry: http-fields/field-names

Sources