title: Finding the min date in a Pandas DF row and create new Column
body: <p>I have a table with a number of dates (some dates will be NaN) and I need to find the oldest date
  so a row may have DATE_MODIFIED, WITHDRAWN_DATE, SOLD_DATE, STATUS_DATE  etc..</p>
  
  <p>So for each row there will be a date in one or more of the fields I want to find the oldest of those and make a new column in the dataframe.</p>
  
  <p>Something like this, if I just do one , eg DATE MODIFIED I get a result but when I add the second as below</p>
  
  <pre><code>table['END_DATE']=min([table['DATE_MODIFIED']],[table['SOLD_DATE']])
  </code></pre>
  
  <p>I get:</p>
  
  <pre><code>ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()
  </code></pre>
  
  <p>For that matter will this construct work to find the min date, assuming I create correct date columns initially?</p>