Example 1: The pattern is: if a sequence of events E1 and then E2 occur, derive event E3.
Let's assume that event E1 occurs at 9:00 and arrives to the system at 9:02, and event E2 occurs at 9:30, and arrives to the system at 9:31. The derived event is derived by the system in 9:33. The question is when does event E3 occurs. One can think of three logical possibilities:
I: E3 occurs when it is produced in 9:33; the rationale: since it is a virtual event, it does not occur in reality, and exists only since it is derived by the system.
II: E3 occurs when the last event that triggers the pattern matching occurs, in this case, in 9:30; the rationale: the derived event occurs when the patterns conditions are satisfied in reality, and this occurs when E2 occurs.
III: E3 occurs over the interval [9:00, 9:30]; the rationale: the derived event occurs over the interval of all participating events.
Example 2: The pattern is time-out (absence event). Example: if there is no bid for an auction by the end of the auction time, derive an event "no bidders".
Scenario: A bid was issued in 9:00 and is valid for 2 hours, in 11:00 it is closed without any bidders, in 11:02 the system issues the derived event.
We have similar three alternatives here:
I: The no bidders event occurs in 11:02, the time that the derived event is issued.
II: The no bidders events occurs in 11:00, when the "bid close" event occurs, which completes the pattern.
III: The no bidders event occurs during the interval [9:00, 11:00] --- since the "absent" event relates to the entire interval.
Like some other cases, there is no single solution that fits all cases; and the actual semantics of a specific case is a matter of policies, we see here three policies, which seem to cover most cases, but not necessarily all, that's why there is a need also to enable explicit derivation of the occurrence time of a derived event, i.e. the value of the occurrence time itself can be computed and derived.
More about temporal issues -- later.