It used to be that the geologic time scale was, dare I say, carved in stone. The Cambrian, Ordovician, Silurian and so on marched in their rigorous order, and that's all we needed to know. The exact dates involved were hardly important, since the assignment of an age relied only on fossils. That was a lifetime ago. Today the time scale is in a furious ferment, in the process of being nailed down by isotopic dating methods, and it seems like the International Commission on Stratigraphy updates the official scale every year now.
I have just updated the time scales on this site based on the January 2013 updates. There are no new geologic time units or anything dramatic like that—thank goodness; the angst and kerfuffle over the Quaternary is over. But the ages of the boundaries between time units keep getting fine-tuned and adjusted. Some of the dates have changed by more than 7 million years since 2009, the last version I featured. For instance, now the Cambrian Period begins at 541 million years ago instead of 542. The Permian-Triassic boundary is at 252 Ma instead of the nice round 250 we used to use. So start with the master eon-era time scale and reacquaint yourself with the timeline of geohistory.
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