Recovering the parent struct

A list stores pointers to struct node embedded inside struct timer. Given struct node *n, how does container_of get the timer?

struct node { struct node *next; };
struct timer { uint32_t period; struct node link; };
#define container_of(p, T, m) ((T *)((char *)(p) - offsetof(T, m)))
struct timer *t = container_of(n, struct timer, link);