If-Modified-Since should set only when server have return Last Modified ...

1. By the default in a browser, when server haven't response back the **Last-Modified**, then the browser never need to append **If-Modified-Since** to the next request. 

2. Some application may defined only Etag and need only validate only Etag parameter, to force it set by **Date**, it also force server to revalidate on this parameter and it will match only Etag parameter, while, **If-Modified-Since** force the server to get new data instead.