When did Euler find his formula for $\zeta(2n)$

Question

Does anybody know when Euler found his famous formula $$\zeta(2n)=\frac{(-1)^{n-1}(2\pi)^{2n}B_{2n}}{2(2n)!}?$$

Answer 1

Looks like 1734, according to this article. But he didn't prove it completely until a few years later.

http://www.math.wvu.edu/~mays/745/Euler%20and%20the%20zeta%20function.pdf