I. Spontaneous formation of space crystals II. Spontaneous breaking of continuous time translation symmetry II.1 Wilczek model II.2 Lack of time crystal formation by systems in the ground states II.3 Excited eigenstates and time crystals formation II.4 Superfluid phase and Josephson oscillations and their relation to time crystals III.Discrete time crystals III.1 Proposals of discrete time crystals