Here is the yearly info from the model (line 5216 of MG8M0F1)
      data amonth/'M01', 'M02', 'M03', 'M04', 'M05', 'M06',             8009.15 
     +            'M07', 'M08', 'M09', 'M10', 'M11', 'M12'/             8009.16 
      DATA JDOFM/0,61,127,193,258,318,372,422,468,515,562,613,669/      8012.   
      DATA JDPERY/669/,JMPERY/12/,EDPERY/669/,Z1I/.1/,RHOI/916.6/       8013.   
A year is 16056 Martian hours which are a little longer than Earth hours
since the Martian day is 24 hours and 39 minutes. The GISS GCM is locked
to 24 hours a day,, and 12 months a year. The only option is to use an 
Earth like system that has extra days every month and extra seconds 
every hour.

Days per month are therefore: 61, 66, 66, 65, 60, 54, 50, 46, 47, 47, 51, 56

Note that the 669 number above is Martian Sols per Martian year. It takes 687 Earth days for Mars to orbit from one vernal equinox to the next.