PWA cache policy: timestamp?


Cache problem for a PWA:

I imagine this:

The cached content has its first line => timestamp server & timestamp client.

These timestamps are recorded by the last fetch.

The smartphone only knows the client timestamp when reading the cache, and its longitude may have changed since the last fetch.

How to evaluate the timestamps, if the cache should, for example, not be older than 120 minutes?

Thank you very much for your help.