This is a detailed mathematical model of intracellular mammalian circadian clock developed in the paper  

Jae Kyoung Kim and Daniel B. Forger (2012) "A mechanism for robust circadian timekeeping via balanced stoichiometry", Molecular Systems Biology

The model is available in the format of 

1) XPPAUT: DetailedModel.ode
2) Matlab: DetailedModel.m
3) Mathematica: DetailedModel.nb (The fastest simulation)
4) SBtoolbox2: DetailedModel.txt