The complex time-dependent behavior of two case studies has been studied by performing a 2D numerical simulation. The evolution of the Vajont landslide from 1960 to 1963 catastrophic failure, and of the Mt. de La Saxe rockslide from 2009 to 2012 are examined. Simulations were carried out using the finite difference code FLAC. Firstly, we carried out some shear strength reduction analysis for solving elastoplastic critical parameters of the basal shear band. Afterwards we applied different time-dependent viscous rheological models identifying the Burgers-creep viscoplastic model (Mohr-Coulomb failure criterion coupled with general Burgers-creep model) as the most appropriate one for the simulation of the behaviour of the two phenomena and of large rockslides exhibiting a similar behaviour. Both models have been calibrated and subsequently validated on a long time series of monitoring data.