> if I initially got the key "foo" at time T=00:00:00, this library would re-query the backing system until time T=00:00:60? even if I requery it at T=:01?

From what I understood of the README (10 minute expiry, 1 minute window) only cache requests between 09:00 to 09:59 will trigger a pre-emptive backing fetch.

ie. T0-539 seconds uses the first query (no re-fetch), T540-599 does a pre-emptive re-fetch (as long as no-one else is currently doing that), T600- would do a fetch and start the whole timer again.