What is the basis for rejecting the days in Genesis as literal 24 hour days according to old earth creationists

Old Earth Creationists (OECs) interpret the "days" in Genesis 1 as representing long periods (e.g., millions of years) rather than literal 24-hour days, to align with scientific evidence for an ancient Earth. What scriptural passages and theological arguments do OECs use to support this non-literal interpretation?