Middle-Earth Time

I read in one of the Appendix's in the end of The Return of the King something about how long a day, month, and year is compared to our time, but I didn't quite understand it. Can someone help me grasp that concept a little better?

Also, I recently discovered for myself that Tolkien fit his story in to the making of the earth. Such as how it became round and even Atlantis! (I was surprised to read that ) So when did the events in The Silmarillion take place according to our concept of time?