time axis wrong

The monthly_mean files resulting from atmospheric diagnostics is wrong, and it seems this has been the case for a few months. People (e.g. @vsicardi) have been correcting this manually but this needs to be fixed so that plotting works properly.

for example a 2-month file contains these timesteps: 1960-11-01 00:00:00 2110-01-02 00:00:00

etourign@bscearth346:/esnas/scratch/etourign/t00v> cdo sinfon  /esarchive/exp/ecearth/t00v/monthly_mean/tas_f6h/tas_19900101.nc 
   File format: netCDF
    -1 : Institut Source   Ttype    Levels Num  Gridsize Num Dtype : Parameter name
     1 : unknown  unknown  constant      1   1         1   1  I32  : realization   
     2 : unknown  unknown  instant       1   1    131072   2  F32  : tas           
     3 : unknown  unknown  instant       1   1         2   3  F64  : time_bnds     
   Grid coordinates :
     1 : generic      > size      : dim = 1  nx = 1  ny = 0
     2 : gaussian     > size      : dim = 131072  nx = 512  ny = 256  np = 0
                        longitude : first = 0  last = 359.296875  inc = 0.703125  degrees_east  circular
                        latitude  : first = 89.4628216  last = -89.4628216  degrees_north
     3 : generic      > size      : dim = 2  nx = 2  ny = 0
   Vertical coordinates :
     1 : surface                  : 0 
   Time coordinate :  2 steps
     RefTime =  1950-01-01 00:00:00  Units = days  Calendar = standard
  YYYY-MM-DD hh:mm:ss  YYYY-MM-DD hh:mm:ss  YYYY-MM-DD hh:mm:ss  YYYY-MM-DD hh:mm:ss
  1960-11-01 00:00:00  2110-01-02 00:00:00