This request header contains the last modification time of a fragment that is being reloaded.
The server can use the header value to prevent unnecessary re-rendering of unchanged content. See Conditional requests for details and examples.
The header is only set when Unpoly knows the last modification time of the loading fragment.
For this the fragment must have been rendered with an
Last-Modified response header or
The header value is a date/time in RFC 1123 format:
If-Modified-Since: Wed, 15 Nov 2000 13:11:22 GMT