  Discrete time crystalline (DTC) phases have attracted significant theoretical
and experimental attention in the last few years. Such systems require a
seemingly impossible combination of non-adiabatic driving and a finite-entropy
long-time state, which is, however, possible in non-ergodic systems. Previous
works have often relied on disorder for the required non-ergodicity; here, we
describe the construction of a discrete time crystal (DTC) phase in
non-disordered, non-integrable Ising-type systems. After discussing the
conditions for interacting and periodically driven systems to display such
phases in general, we propose a concrete model and then provide approximate
analytical arguments and direct numerical evidence that it satisfies the
conditions and displays a DTC phase robust to local periodic perturbations.
