The first of the Biblical days lasted 24 hours, viewed from the "beginning of time perspective." But the duration from our perspective was 8 billion years.
The second day, from the Bible's perspective lasted 24 hours. From our perspective it lasted half of the previous day, 4 billion years.
The third 24 hour day also included half of the previous day, 2 billion years.
The fourth 24 hour day -- one billion years.
The fifth 24 hour day -- one-half billion years.
The sixth 24 hour day -- one-quarter billion years.
When you add up the Six Days, you get the age of the universe at 15 and 3/4 billion years. The same as modern cosmology. Is it by chance?
I don't know about 'chance" but as a Christian, who is also a Scientist, I found this an interesting way of reconciling the two timelines.
It's the best explanation I ever read. And it seems to make perfect sense.
Yet 24 hours is a somewhat arbitrary amount. I recently read somewhere that 1.5 billion years ago, a "day" on Earth was quite a bit shorter - something like 11 hours!