What is an "up to date" version?

Hi,

I'm implementing a cache for WebID graphs to improve performance. I was
looking at WebID-TLS 4.1 Authentication sequence, which says:
"If the WebID Verifier has an up to date version of the graph in its graph
cache then it can return it."
https://dvcs.w3.org/hg/WebID/raw-file/tip/spec/tls-respec.html#authentication-sequence

The phrase "up to date" is repeated multiple times later on, but not
defined anywhere.

So what is it and who decides that -- the implementor? For how long am I
supposed to safely cache the graphs?


Martynas
atomgraph.com