There are various ways Christians deal with the conclusions of modern science, particularly the difference between the biblical age of the universe (which isn’t a science book) and the apparent age of the universe from science. Millard J. Erickson, author of the thirteen-hundred-page Systematic Theology textbook Christian Theology defines the “Ideal-Time” theory as:
The ideal-time theory says that God created the world in a six-day period a relatively short time ago, but that he made it as if it were billions of years old. This is a genuinely novel and ingenious view. Adam, of course, did not begin his life as a newborn baby. At any point in his life he must have had an apparent (or ideal) age many years older than his actual age (i.e., the number of years since his creation). The ideal-time theory extends this principle. If God created trees, rather than merely tree seeds, they presumably had rings indicating an ideal age rather than their real age. Thus, each element of creation must have begun somewhere in the life cycle. 
This is one of five options Erickson details in this section of his book. He then weighs the merits of each and dismisses the ideal-time theory in favor of the age-day theory . His dismissal of this option has bounced around in my head, unable to find a home, ever since I took this class at seminary. Here are his words:
The ideal-time theory is ingenious and in many ways irrefutable both scientifically and exegetically, but presents the theological problem that it makes God an apparent deceiver. 
I’ve always found it troubling that Erickson could reason away such an “ingenious” position so simply. First, there are at least 101 reasons  to think the Earth may be younger than the postulated 4.6 billion years. Secondly, the age-day theory still doesn’t clear God of the charge of “theological deception”. It merely makes His Word seem deceptive rather than our potentially flawed observations of the universe. Finally, deception is a pretty hefty charge. Who is it that God would be deceiving–the believer or the unbeliever? Man was never intended to come to faith in God by observation of the universe alone (the universe only points to him–hence, the reason most nations and peoples have some belief in a god or many gods). And if man disbelieves merely due to observation of the universe then he is self-deceived (as many are today). The deception is not at the hand of God, rather it is in over-estimating man’s ability to determine Truth.
In fact, to state that God created the universe in the same manner he created Adam, with ideal age, clears up any perceived deception for the believer. There is no discrepancy, in fact, we really shouldn’t expect it to be any other way. Modern cosmology leads us to believe that a young universe is dense and hot. It took several billion years for our sun to cool enough for the Earth to contain water, an essential for life, in its middle state (liquid). So if God was going to create a universe that was necessarily old, but do it in a very short period of time, so that He could get on with the real reason for creation–Man–it would be deceiving only if He didn’t tell us about how he did it. But He did tell us.
Erickson has written a widely used and valuable book for students of theology. But in this case, his dismissal of the ideal-time theory on the grounds that it makes God appear deceptive, is illogical, especially to do so in favor of a theory that makes God’s Word appear deceptive.
 Millard J. Erickson, Christian Theology 2d Ed. (Grand Rapids, MI: Baker Academic, 2007), 406-407.
 Erickson says the age-day theory “is based upon the fact that the Hebrew word (yom), while it most frequently means a twenty-four-hour period is not limited to that meaning. It can also mean epochs or longer periods of time, and that is how it should be understood in this context.” He prefers it because, it “fits well with the geological record.”
 Millard J. Erickson, Christian Theology 2d Ed. (Grand Rapids, MI: Baker Academic, 2007), 407.
 Don Batten, “101 evidences for a young age of the earth and the universe” (www.creation.com), published: 4 June 2009