So He created time in order to be able to create?
That doesn't follow. I don't know "why" God created time, but creation of things is a physical act which, in this case, consumed time. God does not tell us that He created all things in an instant, which He could have done. Instead, He tells us that time transpired during His creation.
Did God know the world "before" He created it? If so, then His knowledge is not the same as existence but some idea that has to be created (in time)?
Yes, I think you're really getting to the heart of the matter here. Does God's knowledge of His certain plan in every detail "count" as existence, or did God create a "lockbox-o-souls" first and then dish them out as He created physical bodies in time. I'm not learned in the theology on this, but I would tend away from the latter.
Before the foundations, God had every knowledge of me that there is, everything that makes me different from everyone else. This He knew from the beginning. Given the absoluteness and concreteness of the certainty that I was going to come into being within time, perhaps there is more than one way to characterize this. To me, that certainty is good enough to equate with reality.
FK: "Yes, after 6 "days", His creation work was perfect and complete, i.e. it wasn't so after the third day."
Then God is subject to time.
How so? As I said, God could have created everything in an instant, but He chose to do otherwise. That makes Him the master of time.
Beginning of what?
If time did not exist, then nothing existed before. It's meaningless to speak of beginning when ti comes to God.
One of the reasons Whitehead gets read at all is that it's hard to think about an unchanging GHHITS (Great Hoo Hoo in the Sky - or god of the philosophers) who has any kind of relation with things that change but does not change itself. In other words, if you're confused, you're in some very high-falutin' company.
I haven't read Augustine on time - though I must have years ago since he talks about it in the Confession. I sure don't remember what he says.