title: Pandas pivot_table issues FutureWarning about inferring datetime64 when using margins on dataset with date type
body: <p>When creating a pivot table that includes a date field in the index I get a <code>FutureWarning</code> error:</p>
  <pre><code>&lt;ipython-input-22-330e19e65ef9&gt;:1: FutureWarning: Inferring datetime64[ns] from 
  data containing strings is deprecated and will be removed in a future version. 
  To retain the old behavior explicitly pass Series(data, dtype={value.dtype})
    pivot_report = pd.pivot_table(
  </code></pre>
  <p>Here's the code that triggers it:</p>
  <pre><code>pivot_report = pd.pivot_table(
      data.loc[(data[&quot;year&quot;] == year_to_report)], 
      index = [&quot;Category&quot;, &quot;Subcategory&quot;, &quot;date&quot;], 
      values = [ &quot;amount&quot;], 
      aggfunc = [np.sum], 
      margins = True)
  pivot_report
  </code></pre>
  <p>The data was read from csv with explicit types, and the <code>date</code> field appears to have been correctly converted to <code>datetime64</code> before it hits the <code>pivot_table</code> call, so I'm not clear why it is complaining about type inference.</p>
  <pre><code>data['date']
  0       2021-09-10
  1       2021-09-08
  2       2021-09-08
  3       2021-09-08
  4       2021-09-08
             ...    
  37299   2008-07-31
  37300   2008-07-31
  37301   2008-07-31
  37302   2008-07-31
  37303   2008-07-31
  Name: date, Length: 37304, dtype: datetime64[ns]
  </code></pre>
  <p>However I note that while the <code>margins</code> parameter adds an <code>All</code> row at the bottom as expected, showing the grand total for the <code>amount</code> field, it also shows what looks like a <code>NaT</code> total for the <code>date</code> field.</p>
  <pre><code>                                                       sum
                                                      amount
  Category         Subcategory        date                  
  Auto &amp; Transport Advertising        2021-01-01        0.00
                                      2021-01-02        0.00
                                      2021-01-03        0.00
                                      2021-01-04        0.00
                                      2021-01-05        0.00
  ...                                                    ...
  Vacation         Withdrawal         2021-09-06        0.00
                                      2021-09-07        0.00
                                      2021-09-08        0.00
                                      2021-09-10        0.00
  All                                 NaT             134.31
  
  [928450 rows x 1 columns]
  </code></pre>
  <p>Is this expected? Why would <code>margins</code> be trying to provide totals for a date field? And would this explain the type inference <code>FutureWarning</code> message?</p>