The Precambrian. Here is a handy table of all the geologic time periods divided up by the Geological Society of America: http://www.geosociety.org/science/timescale/timescl.pdf Notice the far right column? That's the Precambrian. read more
Take a closer look, the year scales will be good. Yup, you read it right, the Precambrian spans from when we figure the earth was formed at about 4.5Ga (billion years) to about 541Ma (million years). That's roughly 3.9 billion years covered by one Eon or about 87 percent of theEarth’s geologic history. read more
The geologic time scale is a system of chronological dating that relates geological strata to time. It is used by geologists, paleontologists, and other Earth scientists to describe the timing and relationships of events that have occurred during Earth's history. The table of geologic time spans, presented here, agree with the nomenclature, dates and standard color codes set forth by the International Commission on Stratigraphy. read more