After refers to a past space of time, while in refers to a future space of time.
Thus, we cannot say, 'He will come after a few days', 'He died in a few days.'-The correct sentences will be, 'He will come in a few days,' and "He died after a few days,' respectively.
To denote a point of time, after is used with all tenses; as, He will come or comes or came after 5 o'clock.
Within also refers to a space of future time; but, whereas in denotes at the end of, within denotes some time before the end of. I shall go in a week=after a week has ended. I shail go within a week =before a week has ended.
At refers to a definite point of time; as, 'He came at noon.
By refers to a point of future time and means 'some time before that': as, 'You must be back by 5 o'clock.'
We reached the place in the same time = we took the same period of time (say, 5 hours) to reach the place; we might have started, and arrived at the destination, at different hours.
We reached the place at the same time = say, 5 o'clock; we might have started at different hours. We reached the place within the same time = say, 5 hours; it may be, one took 3 hours, another 4 hours etc.
We reached the place by the same time say, 5 o'clock, a little earlier, at a short interval from one another.
