code: def datetime_to_unix_timestamp(datetime_object):
    
  
    # Is 'datetime_object' a datetime.datetime() object?
    # Raise 'securesystemslib.exceptions.FormatError' if not.
    if not isinstance(datetime_object, datetime.datetime):
      message = repr(datetime_object) + ' is not a datetime.datetime() object.'
      raise securesystemslib.exceptions.FormatError(message)
  
    unix_timestamp = calendar.timegm(datetime_object.timetuple())
  
    return unix_timestamp
docs: <Purpose>
      Convert 'datetime_object' (in datetime.datetime()) format) to a Unix/POSIX
      timestamp.  For example, Python's time.time() returns a Unix timestamp, and
      includes the number of microseconds.  'datetime_object' is converted to UTC.
  
      >>> datetime_object = datetime.datetime(1985, 10, 26, 1, 22)
      >>> timestamp = datetime_to_unix_timestamp(datetime_object)
      >>> timestamp
      499137720
  
    <Arguments>
      datetime_object:
        The datetime.datetime() object to convert to a Unix timestamp.
  
    <Exceptions>
      securesystemslib.exceptions.FormatError, if 'datetime_object' is not a
      datetime.datetime() object.
  
    <Side Effects>
      None.
  
    <Returns>
      A unix (posix) timestamp (e.g., 499137660).