how to distinguish the different usage of these two? 시간이 ...
시간이 지남에 따라 and 시간이 흐름에 따라 are about the same in meaning. For 시간, 지나다(to pass) is more straightforward and to the point, while 흐르다(to flow) seems to imply the mysterious way time goes by (i.e. slightly more poetic).