How can ‘one billion years’ be a mythological unit of time?
Even if you argue that the Universe is not that old do keep in mind that God is Eternal and Immemorial and that God has always existed.
He existed one billion years ago and He existed a billion billion years ago. He will exist into time beyond our comprehension or our ability to measure.
For our God one billion years is not mythological, it is but a footnote.
If time (as in “space-time”) was part of creation, then time itself did not exist prior to creation. As a result, it is meaningless to attribute *any* length or period of time to God’s existence “prior to” creation (a statement which itself is only a bone thrown to our necessarily time-centered frame of reference).