Constant SETTLED 

pub const SETTLED: TimeDelta;

How long before the response that read it an item must last have changed for its token to be given.

GitHub aborts a GraphQL query that runs past ten seconds, so the state a response reports was read at most about ten seconds before the Date it is stamped with. Sixty seconds is that bound with a wide margin: an item whose later time is further back than this was last changed in an earlier second than the read, so no change after the read can share its second.