def get_reference_data(<EOL>self,<EOL>modified_since: Optional[datetime.datetime] = None<EOL>) -> GetReferenceDataResponse:
if modified_since is None:<EOL><INDENT>modified_since = datetime.datetime(year=<NUM_LIT>, month=<NUM_LIT:1>, day=<NUM_LIT:1>)<EOL><DEDENT>response = requests.get(<EOL>'<STR_LIT>'.format(API_URL_BASE),<EOL>headers={<EOL>'<STR_LIT>': self._format_dt(modified_since),<EOL>**self._get_headers(),<EOL>},<EOL>timeout=self._timeout,<EOL>)<EOL>if not response.ok:<EOL><INDENT>raise FuelCheckError.create(response)<EOL><DEDENT>return GetReferenceDataResponse.deserialize(response.json())<EOL>
Fetches API reference data.

:param modified_since: The response will be empty if no
changes have been made to the reference data since this
timestamp, otherwise all reference data will be returned.