This question must be at least 3 sentences long:

Discuss the geologic time scale. How are time units determined in the Precambrian? How does this differ with time units of the Phanerozoic Eon? Be sure to include why the boundaries and units of the geologic time scale have been modified since the time scale's original inception.

It's a chronological time scale which shows the dating of rocks, and other events that took place on earth during that period. Primarily divided into eons, from the Haden to Archean to Proterozoic to Phanerozoic. All three are called collectively as Precambrian supereon and eon is divided into eras, period and epochs.

Precambrian starte about 4600 million years ago and ended in 500 million years ago. Based on carbon dating of rocks found 3.8 billion years ago life forms started to emerge, and evidence of plate tectonics motion can be 4280 million ago.

As compared to the phanerozoic era which started from 500 million years ago to present studied as an era of diverse land and aquatic plants and animals. Subdivided as Paleozoic Era, Cambrian Period, Ordovician Period,Silurian Period, Devonian Period, Carboniferous Period and Permian Period.

The original time scale has been altered to fit the scientist dating of rocks and geologic evidence that they have collected and is modified from time to time upon newer discoveries.