With the way Old Earthers cite 2 Peter 3:8, you would think their Bibles consist only of that and a few verses from Genesis 1. It is a mantra they repeat ad nauseum: With the Lord, a day is as a thousand years. With the Lord, a day is as a thousand years. With the Lord, a day is as a thousand years. Never mind that the context of that verse (which immediately follows warnings that people will be deliberately ignorant of creation and the global flood) is that God can afford to be patient because He does not experience time as we do. No, we absolutely must interpret 2 Peter 3:8 as meaning "oh, by the way, when God says He created the Earth in 6 days, He didn't mean it literally."
But this interpretation has a serious pitfall. See, either the Creationists are correct in understanding this in light of the return of Christ, or it really is a definition of a day in God's eyes. The problem? If it is a definition of a day, then the thousand years must be taken literally, since the whole point of a definition is to remove ambiguity, not add to it. It would be absurd to think God would include a verse in the Bible that effectively comes down to "an ambiguous amount of time in the eyes of the Lord is an ambiguous amount of time".
So, what would happen if we applied the definition of a thousand years to the days in Genesis? Well, first of all, we must assume that the original plants were ridiculously robust, because they survived 1,000 years without ever once seeing the sun, nor were there any animals to aid in their reproductive cycle. But more importantly, all this does is double the age of the Earth. If the days in Genesis are literal days, the Earth is around 6,000 years old plus 6 days. If the days in Genesis are 1,000 years, that's 6,000 years from the start of Creation week to finish + 6,000 years from then until now. That means the Earth, rather than being 6,000 years old, is actually 12,000 years old.
So, what exactly do Old Earthers hope to gain from this? Really, all this does is make a "young" Earth slightly older. Of course, this is still very poor Bible study. The days in Genesis are literal days. This is quite firmly established. But if we grant the sloppiness, what do we have? Basically nothing. Nothing that would directly harm the "young" earth interpretation, nothing that would support the old earth interpretation, we just have an extra 6,000 years to account for. Of course, that would add a few silly things like 1,000 year old fish, birds etc., and we'd have to say Adam was created pretty close to the end of the 6,000th year, since we know he was not 1,000 years old when he died, but again, this is nothing that wouldn't fit the "young" earth view. We'd just have a bit of silliness to account for.
The obvious solution to this is to just take the Bible as it is written. Nowhere in the entire Bible does God say "by the way, when I said I created the heavens, the earth, and all that is in them in 6 days, I didn't actually mean that". What is written in scripture is that in 6 days, God created the heavens, the earth, and all that is in them. This happened around 6,000 years ago. Old Earthers, give it up, for Christ's sake. Literally, for Christ's sake. We will not earn any points with God for twisting His word, nor will we gain respect in the eyes of the world by telling them "don't believe our Bible, we don't believe it either". Choose this day whom you will serve. As for me and my ministry, we will serve the Lord.