  By analogy with the formation of space crystals, crystalline structures can
also appear in the time domain. While in the case of space crystals we often
ask about periodic arrangements of atoms in space at a moment of a detection,
in time crystals the role of space and time is exchanged. That is, we fix a
space point and ask if the probability density for detection of a system at
this point behaves periodically in time. Here, we show that in periodically
driven systems it is possible to realize topological insulators, which can be
observed in time. The bulk-edge correspondence is related to the edge in time,
where edge states localize. We focus on two examples: Su-Schrieffer-Heeger
(SSH) model in time and Bose Haldane insulator which emerges in the dynamics of
a periodically driven many-body system.
