I’ve found a curious question which confused me a lot. I have a html page contains an image url which hosted on cdn server,on first request (just a little before I uploaded the image) ,the response headers looks like below

in 180 seconds,requsts reponse look like below

after 180 seconds,I clear browser cache and request again,which response code is 200,looks normal

but next request looks confused, which response code is 304

My question, why browser think the cache is expired ,did the expire time equals the reponse header “Date” plus “Cache-Control:max-age”?