By far one of the most misquoted verses in the origins debate within the Church is 2 Peter 3:8: "But, beloved, do not forget this one thing, that with the Lord one day is as a thousand years, and a thousand years as one day." ("and a thousand years as one day" usually being omitted because it doesn't fit the intended point). It is argued that this verse casts doubt on the literal nature of the word "day" in Genesis 1 because God sees time differently.
However, 2 Peter 3:8 is not intended to set a precedent for defining the word "day". In fact, ironically, the surrounding verses talk about people who deliberately forget the things described in Genesis. The whole passage, far from trying to say "Genesis has been misunderstood for 1,400 years", is actually trying to explain why it appears Jesus is being slow in returning. It's because His perspective is different to ours.
But while 2 Peter 3:8 is not intended to define day, Genesis 1:5 does. God calls the light "day" and the darkness "night". Then He says the evening and the morning were the first day. This gives us three elements with which to understand "day" in the proceeding chapter. A "day" is the span between evening and morning, particularly the lighter portion of that time.
It was never God's intention to leave Genesis open for interpretation. He is a wonderful communicator, able to speak to us in a way we can understand. Indeed, His word is designed to give understanding to the simple (Psalm 119:130). Though God's perspective is different to ours, He communicates with us on a level we can understand, using our terms to convey His messages. Therefore, we do not need to read every verse as if God is thinking something entirely different than we would understand from His words. And indeed, this is the only area of scripture where people are daft enough to apply such a ridiculous method of interpretation. No, God knows how to speak to us, and when He does speak to us, we better listen. When God spoke to the Hebrew culture and used normal Hebrew language to convey a literal 6 day creation followed by a literal day of rest, which the Hebrews were intended to mimic in their own work week, He fully intended us to use our perspective of time.