The differences between the Young-Earth interpretation of Genesis and modern scientific theories are nontrivial: the Young-Earth interpretation says that everything in the Universe and on Earth was created in six 24-hour days (with a seventh day of rest), estimated by them to have occurred some 6,000 years ago; whereas recent mainstream scientific theories put the age of the Universe at 13.7 billion years and that of the Earth at 4.6 billion years, with various forms of life, including humans, being formed continually thereafter.
The Day-Age Theory tries to reconcile these views by arguing that the Creation "days" were not ordinary 24-hour days, but actually lasted for long periods of time—or as the theory's name implies: the "days" each lasted an age. According to this view, the sequence and duration of the Creation "days" is representative or symbolic of the sequence and duration of events that scientists theorize to have happened, such that Genesis can be read as a summary of modern science, simplified for the benefit of pre-scientific humans.
Scottish lawyer and geologist Charles Lyell published his famous and influential work Principles of Geology in 1830-1833 which interpreted geologic change as the steady accumulation of minute changes over enormously long spans of time and that natural processes, uniformly applied over the length of that existence (uniformitarianism), could account for what men saw and studied in creation.
In the mid 19th century, American geologist Arnold Guyot sought to harmonise science and scripture by interpreting the "days" of Genesis 1 as epochs in cosmic history. Similar views were held by a protégé of Lyell, John William Dawson, who was a prominent Canadian geologist and commentator, from an orthodox perspective, on science and religion in the latter part of the 19th century. Dawson was a special creationist, but not a biblical literalist, admitting that the days of creation represented long periods of time, that the Genesis flood was only 'universal' from the narrator's limited perspective, and that it was only humanity, not the Earth itself, that was of recent creation.
American geologist and seminarian George Frederick Wright was originally a leading Christian Darwinist. However reaction against higher criticism in biblical scholarship and the influence of James Dwight Dana led him to become increasingly theologically conservative. By the first decade of the 20th century he joined forces with the emerging fundamentalist movement in advocating against evolution, penning an essay for The Fundamentals entitled "The Passing of Evolution". In these later years Wright believed that the "days" of Genesis represented geological ages and argued for the special creation of several plant and animal species "and at the same time endowed them with the marvellous capacity for variation which we know they possess." His statements on whether there had been a separate special creation of humanity were contradictory.
Probably the most famous day-age creationist was American politician, anti-evolution campaigner and Scopes Trial prosecutor William Jennings Bryan. Unlike many of his conservative followers, Bryan was not a strict biblical literalist, and had no objection to "evolution before man but for the fact that a concession as to the truth of evolution up to man furnishes our opponents with an argument which they are quick to use, namely, if evolution accounts for all the species up to man, does it not raise a presumption in behalf of evolution to include man?" He considered defining the days in Genesis 1 to be twenty-four hours to be a pro-evolution straw man argument to make attacking creationists easier, and admitted at Scopes that the world was far older than six thousand years, and that the days of creation were probably longer than twenty-four hours each.
American Baptist preacher and anti-evolution campaigner William Bell Riley, "The Grand Old Man of Fundamentalism", founder of the World Christian Fundamentals Association and of the Anti-Evolution League of America was another prominent day-age creationist in the first half of the 20th century, who defended this position in a famous debate with friend and prominent young Earth creationist Harry Rimmer.
They point out that the Hebrew words for father ('ab) and son (ben) can also mean forefather and descendent, respectively, and that the Biblical scripture occasionally "telescopes"genealogies to emphasise the more important ancestors. This, they argue, renders genealogically-based dating of the Creation, such as the Ussher chronology, to be inaccurate.
They admit that yom can mean a twenty-four hour solar day, but argue that it can refer to an indefinitely long period of time. It is in this sense that the word is employed in Genesis 2:4, with a "day" of God's total creation taking place in the course of "days" of creation.