  We consider the time-frequency analysis of a scattered light curve of a
directly imaged exoplanet. We show that the geometric effect due to planetary
obliquity and orbital inclination induce the frequency modulation of the
apparent diurnal periodicity. We construct a model of the frequency modulation
and compare it with the instantaneous frequency extracted from the
pseudo-Wigner distribution of simulated light curves of a cloudless Earth. The
model provides good agreement with the simulated modulation factor, even for
the light curve with Gaussian noise comparable to the signal. Notably, the
shape of the instantaneous frequency is sensitive to the difference between the
prograde, retrograde, and pole-on spin rotations. While our technique requires
the albedo map to be static, it does not need to solve the albedo map of the
planet. The time-frequency analysis is complementary to other methods which
utilize the amplitude modulation. This paper demonstrates the importance of the
frequency domain of the photometric variability for the characterization of
directly imaged exoplanets in future research.
