title: Pandas date parser sets day to current when missing
body: <p>I'm parsing a fixed width file that supplies a month and a year, and i want to create a pandas date from month and year that uses 1 as the day (instead of the current day of the month as Pandas defaults to do).</p>
  
  <pre><code>df = pd.read_fwf("/file", colspecs=colspecs,header=None,names=names, 
  parse_dates= =  {'calendar':['eligYear',"eligMonth"]},keep_date_col=True)
  </code></pre>
  
  <p>Data example:</p>
  
  <pre><code>eligyear eligmonth
  2012     02
  2012     01
  </code></pre>
  
  <p>Ideal outcome where days = 1 instead of the current day of the month, e.g.</p>
  
  <pre><code>calendar
  2012-02-01
  2012-01-01
  </code></pre>