How do you evaluate arctan(0)?

Evaluating $\arctan \left(0\right)$
is equivalent to asking for what value of $\theta$ is
$\tan \left(\theta\right) = 0$ ?
A brief consideration of what is meant by the $\tan$ function should be sufficient to realize
$\theta \epsilon \left\{0 , \pi , 2 \pi , 3 \pi , \ldots\right\}$