Glossary
Clock Skew
Clock skew is the difference between the clocks of two systems that both need to agree, at least approximately, on the current time — a token issuer and a verifier, for instance. Most authorization mechanisms that depend on time, such as an expiring token, assume both sides are reasonably synchronized; when they're not, the consequences run in both directions: a still-valid token can be rejected as expired, or, more concerning, an actually-expired token can still be accepted as valid because the verifier's clock hasn't caught up yet.
Because the failure is intermittent and tied to infrastructure that normally works quietly in the background, clock skew is an easy dependency to overlook when reasoning about why a time-based authorization check might behave inconsistently. Systems that depend on tight time-based expiry windows need to explicitly account for a reasonable tolerance, rather than assuming perfect synchronization that in practice never quite holds.