Justin Taylor in an excellent article Biblical Reasons to Doubt the Creation Days Were 24-Hour Periods notes.
Contrary to what is often implied or claimed by young-earth
creationists, the Bible nowhere directly teaches the age of the earth.
Rather, it is a deduction from a combination of beliefs, such as (1) Genesis 1:1 is not the actual act of creation but rather a summary of or title over Genesis 1:2-2:3; (2) the creation week of Genesis 1:2-2:3 is referring to the act of creation itself; (3) each “day” (Heb. yom) of the creation week is referring to an 24-hour period of time (reinforced by the statement in Exodus 20:11);
(4) an old-earth geology would necessarily entail microevolution,
hominids, and animal death before the Fall—each of which contradicts
what Scripture tells us; and (5) the approximate age of the earth can be
reconstructed backward from the genealogical time-markers in Genesis.
His main argument is that the creation days may be liken to God’s workday and there is no hint in the bible on how long each of these days are.He also named a few heavy weight theologians who did not believe that the said day is a 24-hour period. Named theologians are Augustine, J. Gresham Machen, Edward J. Young, and Gleason Archer.
I have written from a similar perspective in A Critical Evaluation of Genesis One. Recently I have been listening to an audio book The Sixth Extinction by Elizabeth Kolbert while walking on my treadmill (yes to you skeptics, I do get get some exercise). What strike me is the long history of events on earth that dates back billions of years involving the previous five extinctions of numerous species. Many of these extinctions are associated with global catastrophes, for example a meteor smashing into earth. I find it hard to grasp if all these happened in a 24-hour day period or God will deliberately fool us by embedding all this history into the rocks as suggested in Young Earth creationism.