If a thousand years for us is but a day for God, in God's eyes this should make the earth 7 + 6 = 13 days old, am I right ?
That's not what St. Peter is talking about. Peter is not giving a formula of time for God that converts millennia into days, or days into millennia. Peter is saying time doesn't matter to God, so for us humans for which a single day and a thousand years are vastly different, for God there isn't any difference. So long periods of time matter to us, because we are tiny little mortal men with trivially short lifespans; but God is God--so when God keeps a promise, He keeps His promise, even if from our vantage point it takes a very long time. Peter's underlying point is this: You can trust that Christ will return, this is God's promise and it will happen, even if it is taking a long time from your perspective, nevertheless God will keep His promise: Jesus will return. Trust that.
As far as the age of the earth, don't use the Bible to do geological math. That's not an answer the Bible has, that's like trying to find a recipe for pecan pie in the Bible, it's just not what the Bible is interested in telling us, this is not a question the Bible addresses.
If you want to know how old the earth is you have to start looking at physical stuff to study. If you want to know how old a tree is, you can find that out by counting the tree rings. If you take an ice core sample, you can count the layers of annual melts and freezes to see how long that place has been frozen. You can know what the rate of atomic decay of certain elements are, then study samples of rock or fossils and do the math. Taking various disciplines of study, using a multitude of scientific processes, it is possible to learn how old things are, and work with these to figure out how old our planet is.
Over the last couple centuries, scientists have been learning how to do this, and doing this. And the more we study, the more we learn. At present, given everything we have learned, the scientific consensus is that earth is about 4.5 billion years old. That number might change, as science is always self-correcting, but given current knowledge that is the most accurate information we have available.
-CryptoLutheran