In the first part, we came to the conclusion that the creation account in Genesis should be taken literally. This of course brings up the age of the earth. Last time I looked, almost any way you add them up, all of the genealogies in the Bible add up to approximately 6000 years. Therefore, as a Bible believing Christian, I have to believe that the earth is approximately 6000 years old.
Now, some might rightly wonder, what about all of the scientific studies that claim the oldest rocks on the earth are on the order of 4.5 billion years old (byo). First, almost all of the dating methods start with the assumption that the earth is ancient, and that the various processes producing the elements of interest have never changed. There is no way to truly validate this assumption, nor to verify what the starting conditions in the rocks of interest actually are.
Second, many of the rock dating methods do not agree with each other, and the magnitude of the disagreement can be huge. In addition, there are many other pieces of evidence that do not appear to agree with the earth being as old as claimed (see here for some). One that I find particularly interesting is that there are certain rocks that contain helium gas. The helium in these rocks is leaking out at a known rate, and there is a theoretical maximum amount they could hold. However, if the rocks from which this helium is escaping were really as old as claimed, then there should be absolutely no helium left in them! (see here).
Some might point to the Grand Canyon as evidence for an old earth. The proposition is that the many layers of rock were laid down slowly over millennia, and that subsequently the river carved out the canyon over many more. The eruption of Mount St. Helens has demonstrated on a smaller scale that the processes both of sedimentary layer deposition and erosion can occur extremely quickly, especially during various catastrophes. What bigger geologic catastrophe do we have recorded for us by the very words of God but that of a worldwide flood?
So, even though we must as Christians consider the Bible as a witness to time, there are actually many other attestations to it's veracity in scientific fields, if one does not start with the presupposition that the earth is much older than is claimed by the Bible.