Find $\theta$ from X-Y coordinates from $(-1, -1)$ to $(1, 1)$ around $(0, 0)$

Question

For coordinates from $X \in [-1,1]$, and $Y \in [-1,1]$, how do I find an angle around the origin?

I tried using $tan^{-1}$, but it has issues when $X$ is $0$.

How can I find the angle anywhere in the unit circle?