Since the magnetosheath plasma is highly turbulent, reconnection at the dayside magnetopause is likely to be temporally unsteady. The tearing mode can be viewed as a model for the unsteady development of a reconnecting magnetic topology. Magnetopause tearing occurs in the guide field limit and has a wave packet spatial structure in the east-west direction. We solve for the nonlinear evolution of a single wavelength guide-field tearing mode includig the effects of finite transit time on the Landau resonant electrons. Short wavelength modes evolve algebrically in time with perturbation amplitudes proportional to t2. Long wavelength modes are fully nonlinear, and the amplitude grows linearly in time.