space time misunderstanding

Space time misunderstanding 

We all know that time is unidirectional and if we want to travel in time then we can only travel in future never in past... 

I have an example for it.. 

Let us suppose that there is a student traveling in train back to home.. And train takes two days to reach the destination. 

And let us assume that the student have a twin brother studying in same place but he is returning by plane, and it took only two hours to reach home. 

By this example we can understand many things like the first student and second student are traveling same distance but with different speeds and both took different times.. Second student have much more time to spend with family than first student. But the intresting thing is that when first student reached home then the age of both the students are same despite of their speed variations. Second student only travelled through space not time , he travelled same distance in less time that doesn't make him jump in time forward he only got more time to spend with his family. 

That means if we travel with the speed of light in outer space for some time in the path of earth then it doesn't employ that we have travelled in time. By the time earth reaches us we will be of the same age as that of people's on earth.

We only travelled through space, speed of time is still constant so we travelled in time by the speed of time which is fixed.