Do we have any evidence of eternity? I think angels are the best evidence, they came directly from God. Humans are very different. they come indirectly from God by way of evolution. I know my mother was a human. I am sure angels would say their "mother" is God. That is a fundamental difference between heavenly creatures, and earthly creatures.
According to biblical accounts angels are timeless, or they have no time-related characteristics. Therefore, we can conclude that timelessness, or, if you will, a pure form without blemish or sign of aging, is eternal? Humans, on the other hand, are mortal--they age. So, if an angel, after its creation, lives forever, we can say it is almost, but not quite, eternal. Because God has no beginning or ending, we can say He is 100% eternal.
If you are visited by an angel, you've had a close encounter with eternity.