title: Change time axis units from "years since" to "days since" in netcdf file
body: <p>I have a netcdf file that someone passed me that uses &quot;years since DATE&quot; as the time units:</p>
  <pre><code>    double time(time) ;
      time:standard_name = &quot;time&quot; ;
      time:long_name = &quot;time&quot; ;
      time:calendar = &quot;proleptic_gregorian&quot; ;
      time:axis = &quot;T&quot; ;
      time:units = &quot;years since 2000-1-1 00:00:00&quot; ;
  </code></pre>
  <p>I want to change the units to &quot;Days since&quot; the same base date - Obviously the timevariable needs to be changed to be consistent. I was hoping for a solution using nco or cdo, but I am a bit stumped.  (A python solution would also be okay, but in fact the reason I need to do this is that I can't actually get the file to open with xarray.)</p>