title: Convert existing datetime array to date
body: <p>I have a column of dates.  I want to calculate the number of days between each datetime in the array and a specified date.  </p>
  
  <pre><code>import datetime    
  print(mydates[0])
  </code></pre>
  
  <p>2013-04-30T20:00:00.000000000-0400</p>
  
  <pre><code>print(mydates[0] - datetime.date(2013,9,20))
  
  TypeError: ufunc subtract cannot use operands with types dtype('&lt;M8[ns]') and dtype('O')
  </code></pre>
  
  <p>I need to create an array with the number of days between those dates but, more generally, I will also need to know how to convert an array of datetimes into dates.</p>
  
  <p>I found another post that converted datetime.datetime.now() into a date but I don't know how the syntax would work to use that on a an existing set of date times.</p>
  
  <p><a href="https://stackoverflow.com/questions/3743222/how-do-i-convert-datetime-to-date-in-python">Here is the link</a></p>