title: Reindex a Pandas series finding nearest date
body: <p>I have a Pandas time series that looks like this:</p>
  
  <pre><code>In [1]: ser1
  Out[1]: 
  Date
  2005-12-31    11382000
  Name: Amount, dtype: float64
  </code></pre>
  
  <p>I would like to reindex it, using the index of another time series:</p>
  
  <pre><code>In [2]: ser2
  Out[2]: 
  Date
  2005-12-20    14.13
  2005-12-21    14.22
  2005-12-22    14.30
  2005-12-23    14.35
  2005-12-27    14.32
  2005-12-28    14.32
  2005-12-29    14.23
  2005-12-30    14.19
  2006-01-03    14.48
  2006-01-04    14.54
  2006-01-05    14.68
  Name: Amount, dtype: float64
  </code></pre>
  
  <p>But when I use</p>
  
  <pre><code>ser3 = ser1.reindex(ser2.index)
  </code></pre>
  
  <p>I get</p>
  
  <pre><code>In [4]: ser3
  Out[4]: 
  Date
  2005-12-20   NaN
  2005-12-21   NaN
  2005-12-22   NaN
  2005-12-23   NaN
  2005-12-27   NaN
  2005-12-28   NaN
  2005-12-29   NaN
  2005-12-30   NaN
  2006-01-03   NaN
  2006-01-04   NaN
  2006-01-05   NaN
  Name: Amount, dtype: float64
  </code></pre>
  
  <p>Notice that the item from ser1 having a date of '2005-12-31' does not appear in ser3, because ser2's index did not include 2005-12-31.  I would like to put ser1's values on the next available date in ser2's index.  How can I do this?</p>