CESM supports a 365 day (or no-leap) calendar as well as a gregorial calendar. The calendar is set by the xml variable, CALENDAR, in env_build.xml. The no-leap calendar has the standard 12 months, but it has 365 days every year and 28 days in every February. Monthly averages in CESM are truly computed over varying number of days depending on the month of the year. In CESM1.0.x, a gregorian calendar was only possible if the ESMF library was used. This is no longer the case in CESM1.1.x and CESM1.2.x.