A quasi-polynomial model for day duration deviations from standard ones is constructed for the period 1832—2000 yr. The main components of the model are represented by a nonlinear trend and oscillations with periods of 64, 32, and 21 years. Mathematical descriptions of main oscillations define a periodic process with slowly varying amplitude. The results of two-channel autoregressions analysis are given. For the comparative analysis, some data on characteristics of solar activity, magnetosphere processes, and seismic activity of the Earth were used. The question on possible sources and mechanisms of generation of the main oscillations of day duration is discussed.