The largest unit of defined geologic time is the supereon. Only one is defined, the Precambrian spanning from the formation of the Earth to right before life goes crazy in the Cambrian explosion (4.6 billion years ago to 542 million years ago). Oddly, there is no other supereon after the Precambrian, just the Phanerozoic eon . . . → Read More: On the Reasons Why We Need A New Supereon