Reference for Berakhot 3:14

וממאי דהאי ובא השמש ביאת השמש והאי וטהר טהר יומא

How do we know that this [Scriptural reference], "and the sun would set" (lit.: "and the sun would come") refers to sunset, and "…and it would be pure" means that the day would be cleared?

Jastrow