;With CTE
AS
(
SELECT ROW_NUMBER() OVER (PARTITION BY deviceID, accountID ORDER BY [timestamp]) AS Rn,deviceID, accountID, [timestamp] 
FROM Table
)
SELECT c1.*,c2.[timestamp],c2.[timestamp] - c1.[timestamp] AS Diffstp
FROM CTE c1
LEFT JOIN CTE c2
ON c1.deviceID = c2.deviceID 
AND c1.accountID = c2.accountID 
AND c1.Rn = c2.Rn - 1

select a.accountid
      ,a.deviceid
      ,a.timestamp as timestamp1
      ,min(b.timestamp) as timestamp2
      ,min(b.timestamp)-a.timestamp as diffstp
  from eventdata as a
       left outer join eventdata as b
                    on b.accountid=a.accountid
                   and b.deviceid=a.deviceid
                   and b.timestamp>a.timestamp
 group by a.accountid
         ,a.deviceid
         ,a.timestamp