Day Age Creationism
Frequently the concept of "as day is a thousand years" from 2 Peter 3:8 is used to justify the concept.
1But, beloved, be not ignorant of this one thing, that one day is with the Lord as a thousand years, and a thousand years as one day. 2Peter 3:8 (KJV)
Other verses are also cited, such as Psalms 90:4. The basic point behind this line of reasoning is that God works on timescales beyond that of human experience. Another point from day-age creationism is that ancient people would not have had words to describe lengths of time as long as those claimed by modern geologists. If God used these great lengths of time to form the Earth, the best way to describe this would be by relating His work week to that of the human work week. And in the process, set up the pratice of the weekly Saboth rest.
Agreement with science is an important element of day age creationism. There are many chronologies that have been drawn up, aligning the days of creation in Genesis to ages and critical events in pre-history that have been described through scientific findings.