Times filter

Use:

 WHERE StrTime <= @EndTime

  AND @StrTime <= ISNULL(EndTime,'29991231')

Instead of:

WHERE

(  (StrTime >= @StrTime and StrTime <= @EndTime) -- stoppage started while the given time and finished while or after the given time

   OR (EndTime > @StrTime and EndTime <= @EndTime) -- stoppage finished while the given time and started while or before the given time

   OR (StrTime < @StrTime and EndTime > @EndTime) -- stoppage started before the given time and finished after the given time

   OR (StrTime < @StrTime and EndTime IS NULL)  -- stoppage started before the given time and still not finished

     )