code: def posix_to_dt_str(posix):
    
    dt = datetime.datetime.utcfromtimestamp(posix)
    dt_str = dt.strftime(_DT_FORMAT)
    return dt_str + '.000Z'
docs: Reverse of str_to_datetime.
  
    This is used by GCS stub to generate GET bucket XML response.
  
    Args:
      posix: A float of secs from unix epoch.
  
    Returns:
      A datetime str.