One Day is as a Thousand Years
But, beloved, be not ignorant of this one thing, that one day is with the Lord as a thousand years, and a thousand years as one day.
- 2 Peter 3:8 (KJV)
Based on the above, some have suggested that in Genesis, a day is not a literal day, but a thousand years instead. But even if that were true, Genesis would still be "thousands" of years, and not billions of years.
Most Christians compromise the Genesis account to allow for man-made Big Bang, and Evolution theories (which require millions, and sometimes even billions of years to "work").
But there is no need for a compromise. The scriptures are quite clear.
For in six days the LORD made heaven and earth, the sea, and all that in them is, and rested the seventh day: wherefore the LORD blessed the sabbath day, and hallowed it.
- Exodus 20:11 (KJV)
I think the "one day is as a thousand years" verse just means that when you spend time with God (Holy Spirit), it feels like a long time, but in reality only a short period of time has past.
God Himself is clearly outside of time.