Functional calculus for dual quaternions

We give a formula for $f(\eta)$, where $f :\mathbb C \to \mathbb C$ is a continuously differentiable function satisfying $f(\bar z) = \overline{f(z)}$, and $\eta$ is a dual quaternion. Note this formula is straightforward or well known if $\eta$ is merely a dual number or a quaternion. If one is willing to prove the result only when $f$ is a polynomial, then the methods of this paper are elementary.