code: def dt_str_to_posix(dt_str):
    
    parsable, _ = dt_str.split('.')
    dt = datetime.datetime.strptime(parsable, _DT_FORMAT)
    return calendar.timegm(dt.utctimetuple())
docs: format str to posix.
  
    datetime str is of format %Y-%m-%dT%H:%M:%S.%fZ,
    e.g. 2013-04-12T00:22:27.978Z. According to ISO 8601, T is a separator
    between date and time when they are on the same line.
    Z indicates UTC (zero meridian).
  
    A pointer: http://www.cl.cam.ac.uk/~mgk25/iso-time.html
  
    This is used to parse LastModified node from GCS's GET bucket XML response.
  
    Args:
      dt_str: A datetime str.
  
    Returns:
      A float of secs from unix epoch. By posix definition, epoch is midnight
      1970/1/1 UTC.