all your questions are answered in the documentation: http://www.cesm.ucar.edu/models/cesm1.2/cesm/doc/modelnl/nl_drv.html note that you should only modify the code if you want to do something that is not already built in to the coupler. based on your explanation, you may want to explore the "variable_year" option of orb_mode... which may satisfy your needs w/o modifying code. good luck!