title: Matplotlib xaxis.set_major_formatter doesn't understand year
body: <h2>Information</h2>
  
  <p>I have the below pd.dataframe which is plotted with</p>
  
  <pre><code>ax = dfPivot.plot()
  plt.grid()
  plt.grid(which='minor')
  plt.show()
  
  In [4]: dfPivot.index
  Out[4]:
  DatetimeIndex(['2020-05-26 12:00:33', '2020-05-26 12:02:13',
                 '2020-05-26 12:03:53', '2020-05-26 12:05:33',
                 '2020-05-26 12:07:13', '2020-05-26 12:08:53',
                 '2020-05-26 12:10:33', '2020-05-26 12:12:13',
                 '2020-05-26 12:13:53', '2020-05-26 12:15:33',
                 ...
                 '2020-05-29 09:08:53', '2020-05-29 09:10:33',
                 '2020-05-29 09:12:13', '2020-05-29 09:13:53',
                 '2020-05-29 09:15:33', '2020-05-29 09:17:13',
                 '2020-05-29 09:18:53', '2020-05-29 09:20:33',
                 '2020-05-29 09:22:13', '2020-05-29 09:23:53'],
                dtype='datetime64[ns]', name='DateTime', length=2499, freq=None)
  </code></pre>
  
  <p>It works it's ok.</p>
  
  <p><a href="https://i.stack.imgur.com/TMgFC.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/TMgFC.png" alt="Plot working without "></a></p>
  
  <hr>
  
  <h2>Problem</h2>
  
  <p>I have a problem when I format the X axis with:</p>
  
  <pre><code>myFmt = mdates.DateFormatter("%d/%m/%y %H:%M:%S") # tried with "%x %X" or any other combination, failed.
  ax.xaxis.set_major_formatter(myFmt)
  </code></pre>
  
  <p>python drops a:</p>
  
  <pre><code>  rom_ordinalf
        dt = datetime.datetime.fromordinal(ix).replace(tzinfo=UTC) ValueError: year 4354626 is out of range
  </code></pre>
  
  <p>I don't understand which is being treated different when I use the set_major_formatter(). 
  I tought that this was only aesthetic change but seems to go deeper.</p>
  
  <p>I think that matplotlib is "trying" to find and american type date on my pd.datetime64[ns] and failed but don't know how to procede.</p>
  
  <p>Thanks</p>