title: ValueError: Series lengths must match to compare when matching dates in Pandas
body: <p>I apologize in advance for asking such a basic question but I am stumped.</p>
  
  <p>This is a very simple, dummy example. I'm having some issue matching dates in Pandas and I can't figure out why.</p>
  
  <pre><code>df = pd.DataFrame([[1,'2016-01-01'], 
                     [2,'2016-01-01'],
                     [3,'2016-01-02'],
                     [4,'2016-01-03']],
                     columns=['ID', 'Date'])
  
  df['Date'] = df['Date'].astype('datetime64')
  </code></pre>
  
  <p>Say I want to match row 1 in the above df.<br>
  I know beforehand that I want to match ID <code>1</code>.<br>
  And I know the date I want as well, and as a matter of fact, I'll extract that date directly from row 1 of the df to make it bulletproof.</p>
  
  <pre><code>some_id = 1
  some_date = df.iloc[1:2]['Date']  # gives 2016-01-01
  </code></pre>
  
  <p>So why doesn't this line work to return me row 1??</p>
  
  <pre><code>df[(df['ID']==some_id) &amp; (df['Date'] == some_date)] 
  </code></pre>
  
  <p>Instead I get 
  <code>ValueError: Series lengths must match to compare</code><br>
  which I understand, and makes sense...but leaves me wondering...how else can I compare dates in pandas if I can't compare one to many?</p>