title: Using xarray to open a multi-file dataset when both the files and dataset have a "time" component
body: <p>I'm not sure how to word this question but I hope this example can explain it.</p>
  
  <p>I have a series of netcdf files per day of data.
  Each file contains a time dimension to the data which as a 30 day forecast.</p>
  
  <p>If I read in a year's worth of data using:</p>
  
  <pre><code>data=xarray.open_mfdataset(files, concat_dim='None', autoclose='True')
  </code></pre>
  
  <p>Then I get:</p>
  
  <pre><code>Dimensions:   (None: 365, lat: 110, lon: 100, time: 395)
  </code></pre>
  
  <p>I'm only interested in the value at the time = 0 for each file, i.e. for file = 0, I want time = 0 for file = 360, I want time = 360, etc.</p>
  
  <p>Basically I think what I want to do is only read in the first element of the time component from each file but I can't seem to figure out how to do that with open_mfdataset.</p>
  
  <p>Even just dropping the unwanted values after reading the whole thing in would be fine but I can't seem to figure that out either because of the way open_mfdataset concatenates the dataset.</p>