title: How to create a time array in python for seasonal data
body: <p>I am working with paleoclimate data (536-550 CE) in NetCDF format, which I imported with xarray. The time format is a bit strange:</p>
  
  <pre><code>import xarray as xr 
  ds_tas_01 = xr.open_dataset('ue536a01_temp2_seasmean.nc')
  
  ds_tas_01['time']
  &lt;xarray.DataArray 'time' (time: 61)&gt;
  array([15360215.25, 15360430.75, 15360731.75, 15361031.75, 15370131.75,
         15370430.75, 15370731.75, 15371031.75, 15380131.75, 15380430.75,
         15380731.75, 15381031.75, 15390131.75, 15390430.75, 15390731.75,
         15391031.75, 15400131.75, 15400430.75, 15400731.75, 15401031.75,
         15410131.75, 15410430.75, 15410731.75, 15411031.75, 15420131.75,
         15420430.75, 15420731.75, 15421031.75, 15430131.75, 15430430.75,
         15430731.75, 15431031.75, 15440131.75, 15440430.75, 15440731.75,
         15441031.75, 15450131.75, 15450430.75, 15450731.75, 15451031.75,
         15460131.75, 15460430.75, 15460731.75, 15461031.75, 15470131.75,
         15470430.75, 15470731.75, 15471031.75, 15480131.75, 15480430.75,
         15480731.75, 15481031.75, 15490131.75, 15490430.75, 15490731.75,
         15491031.75, 15500131.75, 15500430.75, 15500731.75, 15501031.75,
         15501231.75])
  Coordinates:
    * time     (time) float64 1.536e+07 1.536e+07 1.536e+07 ... 1.55e+07 1.55e+07
  Attributes:
      standard_name:  time
      bounds:         time_bnds
      units:          day as %Y%m%d.%f
      calendar:       proleptic_gregorian
      axis:           T
  </code></pre>
  
  <p>So I want to make my own time array that I can use to plot the climate data. For monthly data I used:</p>
  
  <pre><code>import numpy as np
  time = np.arange('0536-01-31', '0551-01-31', dtype='datetime64[M]')   
  </code></pre>
  
  <p>which gives me an array with the years and months between those two dates.</p>
  
  <p>now I grouped my data by season using cdo seasmean ('djf', 'mam', jja, 'son') and got 61 values instead of 180. Is there a way to regroup the 'time' array to seasonal values, or create a new time array that corresponds to the seasonal data? </p>