title: Why are NaNs introduced into dimension variable when loading a netcdf file into xarray dataset
body: <p>I am new to xarray so I would like to know if I am doing something wrong.</p>
  
  <p>I have a netcdf file containing three groups (A, B, C) each of which contains several variables having just a time dimension as well as a corresponding 'time_dimension' variable.  The time dimension variable employs Unix timestamps for its values.</p>
  
  <p>In the example below I open the Netcdf file and print out the min and max of the timestamp for each group.  This gives me the expected range of timestamps.</p>
  
  <p>I then load each group from the netcdf file into a xarray dataset using the open_dataset command.  For these datasets I again print out the min and max of the time dimension coordinate timestamp.  The min values are the same as those gotten by directly reading the netcdf file but the max values contain NANs for two of the groups (A and B).</p>
  
  <p>Although I don't show it in the code, the NAN values are all located at the end of the xarray variable values array.  Also, group A contained 4 NAN values while group B contained quite a few more.  Also note that the size of the netcdf variables are the same as the xarray variables for each group.</p>
  
  <p>Does anyone know why NAN values are being introduced into my time dimension coordinates when they are imported into xarray from netcdf?</p>
  
  <p>This is the code I used to demonstrate the problem</p>
  
  <pre><code>import xarray as XR
  from netCDF4 import Dataset
  
  Filename = r'C:\temp\My_data.nc'
  
  #-------------- load netcdf data directly -----------
  
  print('netcdf')  
  
  root = Dataset(Filename,'r',format='NETCDF4')
  grp = root.groups['A']
  dt = grp.variables['time_dimension'][:]
  print('group A: ',min(dt), max(dt))
  
  grp = root.groups['B']
  dt = grp.variables['time_dimension'][:]
  print('group B: ',min(dt), max(dt))
  
  grp = root.groups['C']
  dt = grp.variables['time_dimension'][:]
  print('group C: ',min(dt), max(dt))
  
  root.close()
  
  print('   ')
  print('   ')
  
  #-------------- load netcdf data via xarray -----------
  
  print('xarray loaded from netcdf')
  
  ax = XR.open_dataset(Filename, group='A', decode_times=False)
  dt = ax['time_dimension'].values
  print('group A: ', min(dt), max(dt))
  ax.close()
  
  ax = XR.open_dataset(Filename, group='B', decode_times=False)
  dt = ax['time_dimension'].values
  print('group B: ', min(dt), max(dt))
  ax.close()
  
  ax = XR.open_dataset(Filename, group='C', decode_times=False)
  dt = ax['time_dimension'].values
  print('group C: ', min(dt), max(dt))
  ax.close()
  </code></pre>
  
  <p>This is the output of the above code</p>
  
  <pre><code>netcdf
  group A:  1417532400.0 1480406400.0
  group B:  1392129000.0 1439217000.0
  group C:  1432913400.0 1436888700.0
  
  
  xarray loaded from netcdf
  group A:  1417532400.0 9.96920996839e+36
  group B:  1392129000.0 9.96920996839e+36
  group C:  1432913400.0 1436888700.0
  </code></pre>