title: Editing Parquet files with Python causes errors to datetime format
body: <p>I'm trying to edit a set of <code>parquet</code> files that were written in <code>Spark</code>. I'm loading them into <code>pandas</code> dataframes in Python, using <code>Pyarrow</code>. The goal is to append new rows with some data and output a new set of parquet files.</p>
  
  <p>The issue I'm having is with the date/time column, which after loading the parquet files in python shows up as <code>dtype('&lt;M8[ns]'</code>. After adding rows with new times, and outputting new parquet files, I'm using Amazon Athena to query the files. At this point, the values in this column show up in this format <code>+50975-10-19 00:00:00.000</code>. The correct format should be <code>2019-01-05 00:00:00.000</code></p>
  
  <p>Here is a snippet of the python code that shows the workflow to load, edit and output these parquet files:</p>
  
  <pre><code>import pandas as pd
  import pyarrow as pa
  import pyarrow.parquet as pq
  import numpy as np
  
  df1 = pd.read_parquet('example.snappy.parquet')
  
  df1 = df1.append({'visitor_id' : '1234' , 'visitor_attribute_1' : True} , ignore_index=True)
  
  #entering a new time stamp for above user
  df1['visit_dates'].values[131] = np.datetime64('2019-01-03T00:02:11')
  
  #output new parquet file with new row
  table1 = pa.Table.from_pandas(df1)
  pq.write_table(table1, 'example.snappy.parquet', compression='SNAPPY')
  
  </code></pre>
  
  <p>Any help with this would be really appreciated!</p>