The United States was founded? 4th July 1776.
Columbus reached the Americas in 1492, but it wasn't the US then. There was a whole revolution and everything.
To be even more pedantic, Columbus arrived in South America. He never saw any part of what would become the U.S.
Also we celebrate the birth of our nation on July 4th, the supposed anniversary of the signing of the Declaration of Independence, but it was actually signed over a period of several years and wasn't even the Country's first governing doctrine.
And just because it's a fun fact I like, Florida was a Spanish colony for longer than it's been a U.S. state.