property. The trace of the unitary evolution operator for N spins at time T is related to one of a
non-unitary evolution operator for T spins at time N. We characterize this dual operator by its
spectrum. Using the duality relation we obtain the oscillating part of the density of states for a
large number of spins. Furthermore, the duality relation explains the anomalous short-time
behavior of the spectral form factor previously observed in the literature.