title: How to convert date format string to bool True and others to False?
body: <pre><code>import pandas
  import numpy
  df=pandas.DataFrame({'col1':['a','b','c','b'],'col2':['N','2018-03-12 15:35',numpy.NaN,'2017-06-12 15:35'],'col3':['c','b','b','b']})
  print(df)
  </code></pre>
  
  <p>Output of above script is:  </p>
  
  <pre><code>  col1              col2 col3
  0    a                 N    c
  1    b  2018-03-12 15:35    b
  2    c               NaN    b
  3    b  2017-06-12 15:35    b
  </code></pre>
  
  <p>As to column <code>col2</code>,I want to convert all <code>yyyy-mm-dd hh:mm</code> format string to bool <code>True</code>,others to <code>False</code>,keep <code>Na</code> value as same.<br>
  The expect result as below:  </p>
  
  <pre><code>  col1              col2 col3
  0    a             False    c
  1    b              True    b
  2    c               NaN    b
  3    b              True    b
  </code></pre>
  
  <p>How to do it?
  Thanks in advance!</p>