If-Unmodified-Since

Makes a request conditional on a representation not changing after a given date.

Use it as a coarse optimistic-concurrency guard when an ETag is not available. A failed condition normally produces 412.

httpheaderconditionalconcurrency

Reference (http)

If-Unmodified-Since: Wed, 21 Oct 2015 07:28:00 GMT

Use it as a coarse optimistic-concurrency guard when an ETag is not available. A failed condition normally produces 412.

Common mistakes

Using it as a precise validator when multiple changes can occur within the timestamp resolution.

IANA registry: http-fields/field-names

Sources