title: add offset to the datetime64 column in a data frame
body: <p>this is really a quick one:</p>
  
  <p>i am migrating from q to pandas, i am trying to add 1 nano to each of the item in the Date column of the data frame 'spy'</p>
  
  <pre><code>&gt;&gt;&gt; spy
  &lt;class 'pandas.core.frame.DataFrame'&gt;
  Int64Index: 126 entries, 0 to 125
  Data columns (total 6 columns):
  Date      126  non-null values
  Open      126  non-null values
  High      126  non-null values
  Low       126  non-null values
  Close     126  non-null values
  Volume    126  non-null values
  dtypes: datetime64[ns](1), float64(4), int64(1)
  </code></pre>
  
  <p>for illuatration, i have this 1 nano</p>
  
  <pre><code>ttt=np.datetime64(1,'ns')
  </code></pre>
  
  <p>then i try to do:</p>
  
  <pre><code>[x+ttt for x in spy['Date']]
  </code></pre>
  
  <p>I got the following error:</p>
  
  <pre><code>Traceback (most recent call last):
    File "/tmp/py6868jTH", line 9, in &lt;module&gt;
      [x+ttt for x in spy['Date']]
  TypeError: ufunc add cannot use operands with types dtype('O') and dtype('&lt;M8[ns]')
  </code></pre>
  
  <p>can anyone enlight me what's wrong here? ttt and x should be of the same type, right?</p>
  
  <p>thanks!</p>