Old Earth creationism (OEC) is an umbrella of theological views encompassing certain varieties of creationism which may or can include day-age creationism, gap creationism, progressive creationism, and sometimes theistic evolutionism.
Broadly speaking, OEC usually occupies a middle ground between young Earth creationism (YEC) and theistic evolution (TE). In contrast to YEC, it is typically more compatible with the scientific consensus on the issues of physics, chemistry, geology, and the age of the Earth. However, like YEC and in contrast with TE, some forms of it reject macroevolution, claiming it is biologically untenable and not supported by the fossil record, and the concept of universal descent from a last universal common ancestor.
For a long time Evangelical creationists generally subscribed to Old Earth Creationism until 1960 when John C. Whitcomb and Henry M. Morris published the book The Genesis Flood, which caused the Young Earth creationist view to become prominent.
Augustine interpreted the days of Genesis allegorically, whose view also influenced Gregory the Great, Bede and Isodor of Seville. Augustine was not alone in viewing the days of Genesis as allegorical, others include: Didumyus the Blind, possibly Basil the Great, Clement of Alexandria, Origen and Athanasius, who interpreted the days of the Genesis narrative allegorically.
Cyprian argued that each days of Genesis consisted of 1000 years. Irenaeus and Justin Martyr suggested that the days of Genesis could have been long epochs of 1000 years, quoting Psalm 90:4 and perhaps 2 Peter.
According to Dr. Hugh Ross, Thomas Aquinas also denied the genesis account as being literal with six 24 hour days.
Thomas Chalmers popularized gap creationism, which is a form of Old Earth Creationism. Additionally it was advocated by the Scofield Reference bible, which caused the theory to survive longer.
Probably the most famous day-age creationist was American politician, anti-evolution campaigner and Scopes Trial prosecutor William Jennings Bryan.