Complex sentence: The lower boundary (starting point) has not been officially recognized by the International Commission on Stratigraphy, but it is usually set to 3.8 Ga, at the end of the Hadean Eon.
Split into simpler sentences: The lower boundary (starting point) of 3.8 thousand million years has not been officially recognized by the International Commission on Stratigraphy. The Archean customarily starts at 3.8 Ga--at the end of the Hadean Eon.