This read-only TimeStamp is described simply as the expiration time of this SA, the point at which it will be torn down and require renegotiation. An admin compares this against the current time to anticipate an upcoming rekey and rule out expiration as the cause if a tunnel drops unexpectedly early. For example, an admin seeing a tunnel drop well before this expiration time would look elsewhere (like a network outage) rather than blaming a scheduled SA expiry.