diff --git a/.github/workflows/check.yml b/.github/workflows/check.yml
index de49a27..515659b 100644
--- a/.github/workflows/check.yml
+++ b/.github/workflows/check.yml
@@ -1,4 +1,4 @@
-name: style + docs check
+name: Check style + docs + types
 
 on:
   pull_request:
@@ -12,9 +12,9 @@ jobs:
         working-directory: .
 
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Set up Python 3.10
-        uses: actions/setup-python@v4
+        uses: actions/setup-python@v5
         with:
           python-version: "3.10"
           cache: 'pip'
@@ -22,7 +22,17 @@ jobs:
       - name: Install package with dependencies
         run: pip install -e ".[dev]"
         if: steps.python-cache.outputs.cache-hit != 'true'
+
+      # check code style
       - name: Run black
         run: black src --check --diff
+
+      # check docs
       - name: Check that documentation can be built
         run: tox -e docs
+
+      # check types with mypy
+      - name: Install mypy
+        run: pip install mypy
+      - name: Check types in python src directory; install needed types
+        run: mypy --install-types --non-interactive src
diff --git a/.github/workflows/unit_tests.yml b/.github/workflows/unit_tests.yml
index 15b2668..b9678e9 100644
--- a/.github/workflows/unit_tests.yml
+++ b/.github/workflows/unit_tests.yml
@@ -11,14 +11,14 @@ on:
 
 env:
   # python version used to calculate and submit code coverage
-  COV_PYTHON_VERSION: "3.10"
+  COV_PYTHON_VERSION: "3.11"
 
 jobs:
   python-unit:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python: ["3.8", "3.9", "3.10", "3.11"]
+        python: ["3.9", "3.10", "3.11", "3.12"]
     defaults:
       run:
         working-directory: .
diff --git a/setup.cfg b/setup.cfg
index d716b0e..dc228d6 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -15,10 +15,10 @@ keywords = "dates dating uncertainty uncertain-dates unknown partially-known dig
 classifiers =
     Development Status :: 2 - Pre-Alpha
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9
     Programming Language :: Python :: 3.10
     Programming Language :: Python :: 3.11
+    Programming Language :: Python :: 3.12
     Intended Audience :: Developers
     License :: OSI Approved :: Apache Software License
     Operating System :: OS Independent
@@ -63,7 +63,7 @@ test =
   pytest-ordering
   pytest-cov
 docs =
-  sphinx<7.0.0  
+  sphinx<7.0.0
   sphinx_rtd_theme
   m2r2
 # pin sphinx because 7.0 currently not compatible with rtd theme
@@ -72,15 +72,15 @@ docs =
 where = src
 
 [tox:tox]
-envlist = py38, py39, py310, py311
+envlist = py39, py310, py311, py312
 isolated_build = True
 
 [gh-actions]
 python =
-    3.8: py38
     3.9: py39
     3.10: py310
     3.11: py311
+    3.12: py312
 
 [pytest]
 minversion = 6.0
diff --git a/src/undate/dateformat/base.py b/src/undate/dateformat/base.py
index eafebe5..f4435f4 100644
--- a/src/undate/dateformat/base.py
+++ b/src/undate/dateformat/base.py
@@ -15,7 +15,7 @@
 import importlib
 import logging
 import pkgutil
-from typing import Dict
+from typing import Dict, Type
 from functools import lru_cache  # functools.cache not available until 3.9
 
 
@@ -41,11 +41,12 @@ def to_string(self, undate) -> str:
     # cache import class method to ensure we only import once
     @classmethod
     @lru_cache
-    def import_formatters(cls):
+    def import_formatters(cls) -> int:
         """Import all undate.dateformat formatters
         so that they will be included in available formatters
         even if not explicitly imported. Only import once.
         returns the count of modules imported."""
+
         logger.debug("Loading formatters under undate.dateformat")
         import undate.dateformat
 
@@ -65,7 +66,7 @@ def import_formatters(cls):
         return import_count
 
     @classmethod
-    def available_formatters(cls) -> Dict[str, "BaseDateFormat"]:
+    def available_formatters(cls) -> Dict[str, Type["BaseDateFormat"]]:
         # ensure undate formatters are imported
         cls.import_formatters()
         return {c.name: c for c in cls.__subclasses__()}  # type: ignore
diff --git a/src/undate/dateformat/iso8601.py b/src/undate/dateformat/iso8601.py
index eb7e8eb..f1c5cca 100644
--- a/src/undate/dateformat/iso8601.py
+++ b/src/undate/dateformat/iso8601.py
@@ -28,7 +28,12 @@ def parse(self, value: str) -> Union[Undate, UndateInterval]:
         if len(parts) == 1:
             return self._parse_single_date(parts[0])
         elif len(parts) == 2:
-            return UndateInterval(*[self._parse_single_date(p) for p in parts])
+            # date range; parse both parts and initialize an interval
+            start, end = [self._parse_single_date(p) for p in parts]
+            return UndateInterval(start, end)
+        else:
+            # more than two parts = unexpected input
+            raise ValueError
 
     def _parse_single_date(self, value: str) -> Undate:
         # split single iso date into parts; convert to int or None
diff --git a/src/undate/undate.py b/src/undate/undate.py
index db356f5..ee94bb6 100644
--- a/src/undate/undate.py
+++ b/src/undate/undate.py
@@ -41,14 +41,14 @@ class Undate:
     #: symbol for unknown digits within a date value
     MISSING_DIGIT: str = "X"
 
-    earliest: Union[datetime.date, None] = None
-    latest: Union[datetime.date, None] = None
+    earliest: datetime.date
+    latest: datetime.date
     #: A string to label a specific undate, e.g. "German Unity Date 2022" for Oct. 3, 2022.
     #: Labels are not taken into account when comparing undate objects.
     label: Union[str, None] = None
-    formatter: Union[BaseDateFormat, None] = None
+    formatter: BaseDateFormat
     #: precision of the date (day, month, year, etc.)
-    precision: DatePrecision = None
+    precision: DatePrecision
 
     #: known non-leap year
     NON_LEAP_YEAR: int = 2022
@@ -62,7 +62,7 @@ def __init__(
         label: Optional[str] = None,
     ):
         # keep track of initial values and which values are known
-        self.initial_values: Dict[str, Union[int, str]] = {
+        self.initial_values: Dict[str, Optional[Union[int, str]]] = {
             "year": year,
             "month": month,
             "day": day,
@@ -85,8 +85,8 @@ def __init__(
                 min_year = max_year = year
             except ValueError:
                 # year is a string that can't be converted to int
-                min_year = int(year.replace(self.MISSING_DIGIT, "0"))
-                max_year = int(year.replace(self.MISSING_DIGIT, "9"))
+                min_year = int(str(year).replace(self.MISSING_DIGIT, "0"))
+                max_year = int(str(year).replace(self.MISSING_DIGIT, "9"))
         else:
             min_year = datetime.MINYEAR
             max_year = datetime.MAXYEAR
@@ -111,7 +111,7 @@ def __init__(
             except ValueError:
                 # if not, calculate min/max for missing digits
                 min_month, max_month = self._missing_digit_minmax(
-                    month, min_month, max_month
+                    str(month), min_month, max_month
                 )
 
         # similar to month above — unknown day, but day-level granularity
@@ -128,14 +128,14 @@ def __init__(
             min_day = 1
             # if we know year and month (or max month), calculate exactly
             if year and month:
-                _, max_day = monthrange(year, max_month)
+                _, max_day = monthrange(int(year), max_month)
             elif year is None and month:
                 # If we don't have year and month,
                 # calculate based on a known non-leap year
                 # (better than just setting 31, but still not great)
                 _, max_day = monthrange(self.NON_LEAP_YEAR, max_month)
             else:
-                max_day: int = 31
+                max_day = 31
 
             # if day is partially specified, narrow min/max further
             if day is not None:
@@ -146,9 +146,10 @@ def __init__(
         self.earliest = datetime.date(min_year, min_month, min_day)
         self.latest = datetime.date(max_year, max_month, max_day)
 
-        if not formatter:
-            # TODO subclass definitions not available unless they are imported where Undate() is called
-            formatter = BaseDateFormat.available_formatters()[self.DEFAULT_FORMAT]()
+        if formatter is None:
+            #  import all subclass definitions; initialize the default
+            formatter_cls = BaseDateFormat.available_formatters()[self.DEFAULT_FORMAT]
+            formatter = formatter_cls()
         self.formatter = formatter
 
         self.label = label
@@ -179,9 +180,28 @@ def __repr__(self) -> str:
             return "<Undate '%s' (%s)>" % (self.label, self)
         return "<Undate %s>" % self
 
-    def __eq__(self, other: Union["Undate", datetime.date]) -> bool:
+    def _comparison_type(self, other: object) -> "Undate":
+        """Common logic for type handling in comparison methods.
+        Converts to Undate object if possible, otherwise raises
+        NotImplemented error.  Currently only supports conversion
+        from :class:`datetime.date`
+        """
+
+        # support datetime.date by converting to undate
+        if isinstance(other, datetime.date):
+            other = Undate.from_datetime_date(other)
+
+        # recommended to support comparison with arbitrary objects
+        if not isinstance(other, Undate):
+            return NotImplemented
+
+        return other
+
+    def __eq__(self, other: object) -> bool:
         # Note: assumes label differences don't matter for comparing dates
 
+        other = self._comparison_type(other)
+
         # only a day-precision fully known undate can be equal to a datetime.date
         if isinstance(other, datetime.date):
             return self.earliest == other and self.latest == other
@@ -194,6 +214,7 @@ def __eq__(self, other: Union["Undate", datetime.date]) -> bool:
         )
         # if everything looks the same, check for any unknowns in initial values
         # the same unknown date should NOT be considered equal
+        # (but do we need a different equivalence check for this?)
 
         # NOTE: assumes that partially known values can only be written
         # in one format (i.e. X for missing digits).
@@ -201,12 +222,11 @@ def __eq__(self, other: Union["Undate", datetime.date]) -> bool:
         # internal format for comparison
         if looks_equal and any("X" in str(val) for val in self.initial_values.values()):
             return False
+
         return looks_equal
 
-    def __lt__(self, other: Union["Undate", datetime.date]) -> bool:
-        # support datetime.date by converting to undate
-        if isinstance(other, datetime.date):
-            other = Undate.from_datetime_date(other)
+    def __lt__(self, other: object) -> bool:
+        other = self._comparison_type(other)
 
         # if this date ends before the other date starts,
         # return true (this date is earlier, so it is less)
@@ -235,7 +255,7 @@ def __lt__(self, other: Union["Undate", datetime.date]) -> bool:
         # for any other case (i.e., self == other), return false
         return False
 
-    def __gt__(self, other: Union["Undate", datetime.date]) -> bool:
+    def __gt__(self, other: object) -> bool:
         # define gt ourselves so we can support > comparison with datetime.date,
         # but rely on existing less than implementation.
         # strictly greater than must rule out equals
@@ -244,13 +264,10 @@ def __gt__(self, other: Union["Undate", datetime.date]) -> bool:
     def __le__(self, other: Union["Undate", datetime.date]) -> bool:
         return self == other or self < other
 
-    def __contains__(self, other: Union["Undate", datetime.date]) -> bool:
+    def __contains__(self, other: object) -> bool:
         # if the two dates are strictly equal, don't consider
         # either one as containing the other
-
-        # support comparison with datetime by converting to undate
-        if isinstance(other, datetime.date):
-            other = Undate.from_datetime_date(other)
+        other = self._comparison_type(other)
 
         if self == other:
             return False
@@ -320,7 +337,7 @@ def duration(self) -> datetime.timedelta:
 
     def _missing_digit_minmax(
         self, value: str, min_val: int, max_val: int
-    ) -> (int, int):
+    ) -> tuple[int, int]:
         # given a possible range, calculate min/max values for a string
         # with a missing digit
 
@@ -336,17 +353,17 @@ def _missing_digit_minmax(
         max_match = max(matches)
 
         # split input string into a list so we can update individually
-        min_val = list(value)
-        max_val = list(value)
+        new_min_val = list(value)
+        new_max_val = list(value)
         for i, digit in enumerate(value):
             # replace the corresponding digit with our min and max
             if digit == self.MISSING_DIGIT:
-                min_val[i] = min_match[i]
-                max_val[i] = max_match[i]
+                new_min_val[i] = min_match[i]
+                new_max_val[i] = max_match[i]
 
         # combine the lists of digits back together and convert to int
-        min_val = int("".join(min_val))
-        max_val = int("".join(max_val))
+        min_val = int("".join(new_min_val))
+        max_val = int("".join(new_max_val))
         return (min_val, max_val)
 
 
@@ -362,12 +379,15 @@ class UndateInterval:
     """
 
     # date range between two uncertain dates
+    earliest: Union[Undate, None]
+    latest: Union[Undate, None]
+    label: Union[str, None]
 
     def __init__(
         self,
-        earliest: Union[Undate, None] = None,
-        latest: Union[Undate, None] = None,
-        label: Union[str, None] = None,
+        earliest: Optional[Undate] = None,
+        latest: Optional[Undate] = None,
+        label: Optional[str] = None,
     ):
         # for now, assume takes two undate objects
         self.earliest = earliest
@@ -395,6 +415,10 @@ def duration(self) -> datetime.timedelta:
         """
         # what is the duration of this date range?
 
+        # if range is open-ended, can't calculate
+        if self.earliest is None or self.latest is None:
+            return NotImplemented
+
         # if both years are known, subtract end of range from beginning of start
         if self.latest.known_year and self.earliest.known_year:
             return self.latest.latest - self.earliest.earliest + ONE_DAY
diff --git a/tests/test_dateformat/edtf/test_edtf_transformer.py b/tests/test_dateformat/edtf/test_edtf_transformer.py
index 8dba34d..3271b8b 100644
--- a/tests/test_dateformat/edtf/test_edtf_transformer.py
+++ b/tests/test_dateformat/edtf/test_edtf_transformer.py
@@ -41,4 +41,6 @@ def test_transform(date_string, expected):
     transformer = EDTFTransformer()
     # parse the input string, then transform to undate object
     parsetree = edtf_parser.parse(date_string)
-    assert transformer.transform(parsetree) == expected
+    # since the same unknown date is not considered strictly equal,
+    # compare object representations
+    assert repr(transformer.transform(parsetree)) == repr(expected)
diff --git a/tests/test_undate.py b/tests/test_undate.py
index 723d1a0..cf0d9ce 100644
--- a/tests/test_undate.py
+++ b/tests/test_undate.py
@@ -427,3 +427,6 @@ def test_duration(self):
             Undate(None, 6, 7), Undate(None, 6, 6)
         ).duration()
         assert month_noyear_duration.days == 365
+
+        # duration is not supported for open-ended intervals
+        assert UndateInterval(Undate(2000), None).duration() == NotImplemented