We study a time-periodic non-smooth differential
equation $\dot{x}=f(t,x)$, $x\in \mathbb R$.
In [4] we have presented a sufficient condition for existence, uniqueness,
stability and the basin of attraction of a periodic orbit in such a system,
which is a generalized Borg's condition.
In this paper we prove that this condition is necessary. The proof involves a generalization of
Floquet exponents for periodic orbits of non-smooth differential equations.