Q: How can the mind perceive something which is outside of time, when the mind itself is caught in time like a prisoner? Perception in itself is a movement in time so how can mind even claim to perceive the concept of Brahman and say that Brahman is eternal?
A: The mind cannot perceive anything ‘outside of time’. As you say, perception occurs within time (and space). And, more to the point, that which you perceive is also within time and space. Thus, you can never perceive Brahman. But conceptions are not quite the same. The concept itself is in the mind, which is also limited. But what is conceived is not limited. You can conceive of a unicorn with no problem at all, even though you also know that they do not really exist. Scientists also conceived of black holes, long before any proof was found for their existence – and still no one has ‘really’ perceived such a thing.
But perhaps the simplest way of thinking about it is to consider deeply who you actually are. It is possible to eliminate body, sense organs, mind, and anything else that you can think of as ‘not I’. But it is not possible to eliminate the one who is doing this. There has to be an ‘ultimate subject’ after everything else has been eliminated. That is who you really are and that is Brahman.