If-Modified-Since

The If-Modified-Since request-header field is used with a method to make it conditional: if the requested variant has not been modified since the time specified in this field, an entity will not be returned from the server; instead, a 304 (not modified) response will be returned without any message-body.

       If-Modified-Since = "If-Modified-Since" ":" HTTP-date

An example of the field is:

       If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT

A GET method with an If-Modified-Since header and no Range header requests that the identified entity be transferred only if it has been modified since the date given by the If-Modified-Since header. The algorithm for determining this includes the following cases:

1. If the request would normally result in anything other than a 200 (OK) status, or if the passed If-Modified-Since date is invalid, the response is exactly the same as for a normal GET. A date which is later than the server's current time is invalid.
   
2. If the variant has been modified since the If-Modified-Since date, the response is exactly the same as for a normal GET.
   
3. If the variant has not been modified since a valid If-Modified-Since date, the server SHOULD return a 304 (Not Modified) response.
   

The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead.

Note: The Range request-header field modifies the meaning of If-Modified-Since; see section 14.35 for full details.

Note: If-Modified-Since times are interpreted by the server, whose clock might not be synchronized with the client.

Note: When handling an If-Modified-Since header field, some servers will use an exact date comparison function, rather than a less-than function, for deciding whether to send a 304 (Not Modified) response. To get best results when sending an If-Modified-Since header field for cache validation, clients are advised to use the exact date string received in a previous Last-Modified header field whenever possible.

Note: If a client uses an arbitrary date in the If-Modified-Since header instead of a date taken from the Last-Modified header for the same request, the client should be aware of the fact that this date is interpreted in the server's understanding of time. The client should consider unsynchronized clocks and rounding problems due to the different encodings of time between the client and server. This includes the possibility of race conditions if the document has changed between the time it was first requested and the If-Modified-Since date of a subsequent request, and the possibility of clock-skew-related problems if the If-Modified-Since date is derived from the client's clock without correction to the server's clock. Corrections for different time bases between client and server are at best approximate due to network latency.

The result of a request having both an If-Modified-Since header field and either an If-Match or an If-Unmodified-Since header fields is undefined by this specification.