title: Invalid argument for log file name
body: <p>Here is what I try to do and I use windows and Python 3.3</p>
  
  <pre><code>LOG_FILENAME = 'log_trial_%s.txt' % datetime.datetime.now().strftime('%m%d-%H:%M:%S')
  log_fd = open(LOG_FILENAME, 'w')
  log_fd.write('===================\n')
  </code></pre>
  
  <p>The above codes are of course a part of a module and whe I ran the module, I got an error message as follows:</p>
  
  <pre><code>log_fd = open(LOG_FILENAME, 'w')
  OSError: [Errno 22] Invalid argument: 'log_trial_1209-11:39:40.txt'
  </code></pre>
  
  <p>I have no idea what the long weird log finame means ('log_trial_%s.txt' ...) and how to fix the error thing as well.</p>
  
  <p>I will appreciate your time and help.</p>