My (literal) thoughts on Genesis 1.
Genesis 1:1-2 was presented prior to the active creation of the natural world. The foundation laid in these verses very well may be billions of years old. As we read the account of the natural world coming into existence, time does not exist prior to the third day. So, the first 3 “days” could be of any duration from a nanosecond to millions, billions or trillions of years. When we get past the third day, because time now exists, the days may have been 24 hour days. However, since there were no sentient beings present to interpret the time which passed during days 4-6 even that is in doubt. Time only has meaning when humanity is present to place events into its context.