Are If-Modified-Since and Etag interchangeable? What to use (or both)?

Question

Nginx

At the moment ETag/If-None-Match is configured and working. There are no complaints. But one test here noticed that If-Modified-Since is not used.
Do I need it if I'm already using ETag/If-None-Match? Is it possible to use them together, what are the pitfalls?

Answer 1

Very, very old browsers don't support etag.
You can use either.