docs: def get_timestamp(dt=None):
      '''
      Return current timestamp if @dt is None
      else return timestamp of @dt.
  
      >>> t = datetime.datetime(2015, 0o5, 21)
      >>> get_timestamp(t)
      1432166400
      '''
  
      if dt is None: dt = datetime.datetime.utcnow()
      t = dt.utctimetuple()
  
      return calendar.timegm(t)