--- counts ONLY the time worked between @start and @end 
SELECT    e.emp_id,
    SUM 
    ( 
        DATEDIFF 
        (n,
        CASE WHEN login_time < @start THEN @start ELSE login_time END,
        CASE WHEN logout_time > @end THEN @end ELSE logout_time END
        ) 
    ) 
    AS TotalTimeWithinDateRange
FROM    LoginTable lt 
    INNER JOIN Store s ON lt.store_id = s.store_id
    INNER JOIN Employee e ON lt.emp_id = e.emp_id
WHERE    s.store_id = @store_id AND 
    (
        (lt.login_time BETWEEN @start AND @end) OR
        (lt.logout_time BETWEEN @start AND @end) OR
        (lt.login_time <= @start and logout_time >= @end)
    )
GROUP BY e.emp_id