Not 6 literal days.
Genesis is translated from Hebrew - in that language the word translated into English as 'day' means 'a long time; the time covering an extraordinary event'. It's not 6 literal 24 hour periods (which is where some get confused), it's simply a poetic way of describing the creative periods in a way that humans can understand (consider the original audience - would 'a long time' be more understandable than 'millions or possibly billions of years' to a people who had just been released from generations of slavery?).
Millions of years absolutely fits in with the actual account if the original meaning is referenced.
If it's taken as simply a normal 24 hour day, then the first three creative periods don't make sense, as there was no light and dark (the night and day cycle) until part of the fourth 'day' had already passed (and putting the seasons, days and years in place is in chapter 1, verse 15).
It can't therefore be a day on Earth, as that cycle didn't exist until later.
The word day is also relative - a day on Earth is 24 hours, a day on Venus is roughly 2802 hours. The timescale for the 'day' isn't given, but it's not based on Earth time.