We have discussed what nirvana means in the modern Theravada forum. On the one hand it is simply the point at which a being is totally free of the causes of suffering or rather greed hatred and delusion. On the other hand the Buddha seems to have said that no arising, changeability, or passing away of nirvana will be discerned.
The first way of thinking about nirvana places it in our future. With the right practice conditions there will emerge a state free from greed hatred and delusion then the being who experiences these conditions will become aware of this experience through direct knowledge.
The second way of thinking about nirvana is in terms of it occupying space. In this way it is an expanse of freedom which can be occupied by the being who realizes it. Like a mind which knows that in whatever way it moves through all that can be experienced nirvana is there unchanging.
We can only think of things in terms of either time or space. That is the limitation of thinking.
What view we take of nirvana is really a matter of what helps us to purify our minds. In some cases people who find it difficult to fathom their own ability to work with their minds in a constructive way might be better off thinking of nirvana as something they are looking for in space. Some kind of element which will help them purify their mind. This kind of practice basically amounts to reflecting on the Buddha. On the other hand some people will be more in tune with the changeability of their minds in accordance with conditions and have confidence in their ability to work with these factors though time to move towards purity. I think we all need to work with both ways of viewing nirvana at different times depending on whats happening with us. We are likely to be either one of these types of people at different times.
There is no other way to formulate how we should think about nirvana other than to measure by its effects on our basic practice patterns. Is it helpful in letting go of what binds us in unskillfulness?
These are my thoughts and they are open to change.