Two Clocks, One Response

A response carries Cache-Control: public, max-age=60, s-maxage=600. A CDN edge stores it, and 300 seconds later a browser fetches it and receives the stored copy with Age: 300. How long may that browser treat its own copy as fresh?

Three hundred seconds, the remainder of the s-maxage=600 budget after subtracting the reported age

Zero seconds: the browser applies max-age=60 against an age of 300, so the copy is already stale on arrival

Sixty seconds, because Age describes the CDN's copy and the browser starts its own max-age clock on receipt

Six hundred seconds, because s-maxage overrides max-age for every cache that stores the response