The spatial confinement of light is an important prerequisite for the excitation of light-matter interactions and nonlinear optics. Photonic crystal nanocavities can have very high quality factors that lead to the desired confinement. We study the feasibility of a hybrid time and frequency domain formulation for the computation of the resonance frequencies and the external quality factors of photonic crystal nanocavities. The time domain approach yields estimates for quality factor and resonance frequency by a special postprocessing technique as well as an approximate eigenvector. In the eigenvalue computation the approximate results from time domain analysis are used for the accelerated solution of the eigenvalue problem.