title: Date bug? receiving error only on certain dates
body: <p>The following code gets the start of the week of a date argument:</p>
  
  <pre><code>weekstart = datetime.datetime.strptime(sys.argv[1], '%m/%d/%Y')
  
  if weekstart.strftime('%A') != 'Sunday':
      print "Start date should be a Sunday"
      while weekstart.strftime('%A') != 'Sunday':
          #weekstart = weekstart - datetime.timedelta(a)
          a=a+1
      print "Using %s as start date instead" %weekstart.strftime('%m/%d/%Y')
  </code></pre>
  
  <p>I receive the following error only when the date entered is 7/10/2012, 7/11/2012, 7/12/2012, or 7/13/2012:</p>
  
  <pre><code>while weekstart.strftime('%A') != 'Sunday':
  
  ValueError: year=1899 is before 1900; the datetime strftime() methods require year &gt;= 1900
  </code></pre>
  
  <p>What is wrong with this?</p>