You cannot know that you are timeless and eternal via the mind.
The mind is intermittent.

It is made out of discrete events and cannot reveal that which is not discrete, that which is eternal, that which perceives it.
That which knows all discrete events is not discrete.
It is the I of I am, the I of consciousness, the I of aware presence.
When you look at the mind you perceive intermittent events that arise in time and space appearing in your seamless presence.
It is your seamless presence that is the reality of all intermittent world body mind impressions.
This seamless presence is not personal. All dream characters belong to it but It does not belong to any dream character.