What is "paleoproterozoic"

The Paleoproterozoic (; also Palaeoproterozoic) is the first of the three sub-divisions ( eras) of the Proterozoic occurring (2.5–1.6 Ga). This is when the continents first stabilized.

Paleontological evidence on the Earth's rotational history suggests that ~1.8 billion years ago, there were about 450 days in a year, implying 20-hour days.