title: Pandas Replacing a Null Datetime Value in one df with a Timestamp Value from Another
body: <p>Bug?  If so, suggestions on a work around?</p>
  
  <p>I have the following:</p>
  
  <pre><code>df1['DecisionDate'].head()
  Out[238]: 
  ID
  RED            2017-02-13 00:00:00
  GREEN          2016-07-29 00:00:00
  ORANGE         2017-01-26 00:00:00
  PURPLE         2016-10-31 00:00:00
  YELLOW          NaT
  Name: DecisionDate, dtype: datetime64[ns]
  </code></pre>
  
  <p>And this:</p>
  
  <pre><code>df2['DecisionDate']
  Out[239]: 
  YELLOW   2014-04-05 00:00:00
  Name: DecisionDate, dtype: datetime64[ns]
  </code></pre>
  
  <p>Now if I try to do this:</p>
  
  <pre><code>for ID in df2.index:
  
      df1.ix[ID,'DecisionDate'] = df2.ix[ID,'DecisionDate']
  </code></pre>
  
  <p>I get this</p>
  
  <pre><code>TypeError: long() argument must be a string or a number, not 'Timestamp'
  </code></pre>
  
  <p>No matter what I try I cannot seem to replace the <code>pd.NaT</code> with a timestamp value.</p>
  
  <p>All values in both dataframe are timestamps.  df2 observations are a subset of df1 observations so all values in df2.index are in df1.index.</p>
  
  <p>Am I overlooking something straightforwards?  Or is this a bug?</p>
  
  <p>Thanks in advance for your help.</p>
  
  <p>EDIT</p>
  
  <p>Here is the full traceback.  I'm not good at reading this but maybe it will help in diagnosing:</p>
  
  <pre><code>  File "C:\Users\Anaconda2\lib\site-packages\pandas\core\indexing.py", line 141, in __setitem__
      def _slice(self, obj, axis=0, kind=None):
  
    File "C:\Users\Anaconda2\lib\site-packages\pandas\core\indexing.py", line 533, in _setitem_with_indexer
  
    File "C:\Users\Anaconda2\lib\site-packages\pandas\core\indexing.py", line 473, in setter
      value = getattr(value, 'values', value).ravel()
  
    File "C:\Users\Anaconda2\lib\site-packages\pandas\core\internals.py", line 3168, in setitem
  
    File "C:\Users\Anaconda2\lib\site-packages\pandas\core\internals.py", line 3056, in apply
      align_copy = False
  
    File "C:\Users\Anaconda2\lib\site-packages\pandas\core\internals.py", line 668, in setitem
      def _replace_single(self, *args, **kwargs):
  
    File "C:\Users\Anaconda2\lib\site-packages\pandas\core\internals.py", line 2265, in _try_coerce_args
      Parameters
  
  TypeError: long() argument must be a string or a number, not 'Timestamp'
  </code></pre>