title: pandas to_json and read_json with datetime index
body: <p>I have a <code>pd.DataFrame</code> with index type <code>Timestamp</code>.</p>
  <p>When I <code>to_json</code> the dataframe, the index is converted to something like <code>'1629849600000'</code>.</p>
  <p>I cannot <code>pd.to_datetime</code> this or <code>datetime.datetime.fromtimestamp</code> it.</p>
  <p>If I pd.DataFrame the json, it is well formed:</p>
  <p><a href="https://i.stack.imgur.com/BfjM4.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/BfjM4.png" alt="enter image description here" /></a></p>
  <p>If I <code>pd.to_datetime</code> this, it fails wtih</p>
  <pre><code>OverflowError: Python int too large to convert to C long
  </code></pre>