Transact SQL :: Accumulate Alarm Time Per Event In A Log Table?

I have a log table in which alarm events are logged (a many relation to a main table) where EventId is the foreign key in the log table.

In this table all events related to a given incident (EventID) are logged, but it is specifically two eventtypes which is of interest, thats the "into alarm" represented by an integer value "0" in the column "EventTypeId" and the "restored represented by an integer value of "2". Each state of change has a timestamp of datetime.For each incident (EventId) there can be multiple "into alarm" and "restore" rows, there can also be other events which has no interests. Only the timediff between each "0" and "2" accumulated is of interest.

There's one more little twitch, it can occure that when the query is run, the "2" restored state is not yet present, the query should not return a result for that "0" into alarm when the "2" restored is not present, unless there was a complete alarm/restore session.

CREATE TABLE [dbo].[AlarmHistoryLog](
[EventId] [int] NOT NULL,
[EventSeq] [int] IDENTITY(1,1) NOT NULL,
[EventTypeId] [int] NOT NULL,
[EventTime] [datetime] NOT NULL,
[EventDescription] [nvarchar](250) NOT NULL,
[Comment] [nvarchar](4000) NULL,
[UserID] [int] NULL,
[ReceiverID] [int] NULL
) ON [PRIMARY]