This is a rather simplistic account. Newton and Leibniz neither started nor finished the development of calculus, they just happened to make the connection betw

This is a rather simplistic account. Newton and Leibniz neither started nor finished the development of calculus, they just happened to make the connection between differentiation and integration which opened up the way for the spectacular development of the subject in later years and Leibniz invented some of the notation we still use, but there was lots of progress before and lots of progress since then, especially regarding logical foundations of the subject and a big part of it is now incorporated in the undergraduate curriculum. Linear-algebra based multi-variate calculus is pretty common in universities now too and it wasn't well developed before the 20th century.