def datetime_to_unix_timestamp(datetime_object):
<EOL>if not isinstance(datetime_object, datetime.datetime):<EOL><INDENT>message = repr(datetime_object) + '<STR_LIT>'<EOL>raise securesystemslib.exceptions.FormatError(message)<EOL><DEDENT>unix_timestamp = calendar.timegm(datetime_object.timetuple())<EOL>return unix_timestamp<EOL>
<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).