How to use the elementpath.datatypes.GregorianYear function in elementpath

'name': XSD_DATETIME,
        'python_type': (unicode_type, str, datatypes.DateTime),
        'admitted_facets': DATETIME_FACETS,
        'facets': [COLLAPSE_WHITE_SPACE_ELEMENT],
        'to_python': datatypes.DateTime.fromstring,
    },  # [-][Y*]YYYY-MM-DD[Thh:mm:ss]
    {
        'name': XSD_DATE,
        'python_type': (unicode_type, str, datatypes.Date),
        'admitted_facets': DATETIME_FACETS,
        'facets': [COLLAPSE_WHITE_SPACE_ELEMENT],
        'to_python': datatypes.Date.fromstring,
    },  # [-][Y*]YYYY-MM-DD
    {
        'name': XSD_GYEAR,
        'python_type': (unicode_type, str, datatypes.GregorianYear),
        'admitted_facets': DATETIME_FACETS,
        'facets': [COLLAPSE_WHITE_SPACE_ELEMENT],
        'to_python': datatypes.GregorianYear.fromstring,
    },  # [-][Y*]YYYY
    {
        'name': XSD_GYEAR_MONTH,
        'python_type': (unicode_type, str, datatypes.GregorianYearMonth),
        'admitted_facets': DATETIME_FACETS,
        'facets': [COLLAPSE_WHITE_SPACE_ELEMENT],
        'to_python': datatypes.GregorianYearMonth.fromstring,
    },  # [-][Y*]YYYY-MM
    # --- Datetime derived types (XSD 1.1) ---
    {
        'name': XSD_DATE_TIME_STAMP,
        'python_type': (unicode_type, str),
        'base_type': XSD_DATETIME,

'facets': [COLLAPSE_WHITE_SPACE_ELEMENT],
        'to_python': datatypes.DateTime.fromstring,
    },  # [-][Y*]YYYY-MM-DD[Thh:mm:ss]
    {
        'name': XSD_DATE,
        'python_type': (unicode_type, str, datatypes.Date),
        'admitted_facets': DATETIME_FACETS,
        'facets': [COLLAPSE_WHITE_SPACE_ELEMENT],
        'to_python': datatypes.Date.fromstring,
    },  # [-][Y*]YYYY-MM-DD
    {
        'name': XSD_GYEAR,
        'python_type': (unicode_type, str, datatypes.GregorianYear),
        'admitted_facets': DATETIME_FACETS,
        'facets': [COLLAPSE_WHITE_SPACE_ELEMENT],
        'to_python': datatypes.GregorianYear.fromstring,
    },  # [-][Y*]YYYY
    {
        'name': XSD_GYEAR_MONTH,
        'python_type': (unicode_type, str, datatypes.GregorianYearMonth),
        'admitted_facets': DATETIME_FACETS,
        'facets': [COLLAPSE_WHITE_SPACE_ELEMENT],
        'to_python': datatypes.GregorianYearMonth.fromstring,
    },  # [-][Y*]YYYY-MM
    # --- Datetime derived types (XSD 1.1) ---
    {
        'name': XSD_DATE_TIME_STAMP,
        'python_type': (unicode_type, str),
        'base_type': XSD_DATETIME,
        'to_python': datatypes.DateTime.fromstring,
        'facets': [etree_element(XSD_EXPLICIT_TIMEZONE, value='required')],
    },  # [-][Y*]YYYY-MM-DD[Thh:mm:ss] with required timezone