Why does server send both Etag and date for modifying user?

Hi,

 I was reading about headers that client and server send to each other and I have a question, server and client can use date header to modify server and Etag , my question is this: why does server and client use both Etag and date headers to modifying therefore they can use just one of these ? why does both of these headers were created?

please explain more about these headers and differences of these headers.

thank you very much.