See the answer at https://esgf.github.io/esgf-swt/data/2015/08/13/Do-all-CMIP5-models-use-the-same-calendar.html
The link is dead. These days, the answer can be found here: https://github.com/ESGF/esgf.github.io/wiki/CMIP5HelpDeskFAQ#help-understanding-the-data