Event Time
The time at which the represented event occurred at its source, distinct from the time when a processing system received or handled the record.
Records in a distributed stream need not arrive in the order in which their real-world events occurred. Network delay, retries, or intermittently connected producers can deliver an older event-time record after newer ones.
That distinction changes windowing, ordering, and late-data handling. Processing time is operationally simpler, but event time carries different semantics when records must be grouped according to when the underlying events actually happened. The broader ordering problem appears in concurrency in distributed data systems.