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.