ATHEISTS: Can you justify your unconditional adherence to timeless values without implying the idea of God?

If so, what is the moral rock on which the atheist self-uredness is built? It can't be the human nature, as it is not timeless and clearly whimsical. By suggesting that there are absolutes on which everyone must agree, you are implicitly suggesting the existence of greater goodness. The existence of, you guessed it, God.