How to calculate polar angle of point given a reference point?

Question

I want to calculate polar angle of some points based on different reference points. Usually polar angle is calculated based on reference point (0,0). What is the procedure to calculate polar angle when reference point is (x,y)?

Answer 1

A point (X,Y) with the reference of (0,0) becomes (X-x,Y-y) with reference to (x,y)

Answer 2

To find the polar angle of $(v,w)$ with respect to $(x,y)$, just find the normal polar angle of the vector $(v-x, w-y)$.