def datetime_to_unix_timestamp(datetime_object):
  """
  <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).
  """

  # 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
<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).