diff --git a/ical/list.py b/ical/list.py
index db5b32f..61c982f 100644
--- a/ical/list.py
+++ b/ical/list.py
@@ -36,12 +36,14 @@ class RecurAdapter:
     def __init__(self, todo: Todo, tzinfo: datetime.tzinfo | None = None):
         """Initialize the RecurAdapter."""
         self._todo = todo
+        if todo.computed_duration is None:
+            raise ValueError("Recurring todo must have a duration")
         self._duration = todo.computed_duration
         self._tzinfo = tzinfo
 
     def get(
         self, dtstart: datetime.datetime | datetime.date
-    ) -> SortableItem[datetime.datetime, Todo]:
+    ) -> SortableItem[datetime.datetime | datetime.date | None, Todo]:
         """Return a lazy sortable item."""
 
         recur_id_dt = dtstart
@@ -77,11 +79,11 @@ def _todos_by_uid(todos: list[Todo]) -> dict[str, list[Todo]]:
 
 def _todo_iterable(
     iterable: list[Todo], tzinfo: datetime.tzinfo
-) -> Iterable[SortableItem[datetime.datetime, Todo]]:
+) -> Iterable[SortableItem[datetime.datetime | datetime.date | None, Todo]]:
     """Create a sorted iterable from the list of events."""
 
     def sortable_items() -> (
-        Generator[SortableItem[datetime.datetime, Todo], None, None]
+        Generator[SortableItem[datetime.datetime | datetime.date | None, Todo], None, None]
     ):
         for todo in iterable:
             if todo.recurring:
@@ -111,15 +113,18 @@ def _pick_todo(todos: list[Todo], tzinfo: datetime.tzinfo) -> Todo:
         iters.append(RecurIterable(RecurAdapter(todo, tzinfo=tzinfo).get, recur))
 
     root_iter = MergedIterable(iters)
+    
     # Pick the first todo that hasn't started yet based on its dtstart
     now = datetime.datetime.now(tzinfo)
     last: Todo | None = None
-    todo = next(iter(root_iter))
-    for todo in root_iter:
-        if todo.item.start_datetime > now:
+
+    it = iter(root_iter)
+    last = next(it, None)
+    while cur := next(it, None):
+        if cur.item.start_datetime is None or cur.item.start_datetime > now:
             break
-        last = todo.item
-    return last
+        last = cur
+    return last.item if last is not None else None
 
 
 def todo_list_view(
diff --git a/ical/todo.py b/ical/todo.py
index 28f9d02..a8baea2 100644
--- a/ical/todo.py
+++ b/ical/todo.py
@@ -89,15 +89,6 @@ class Todo(ComponentModel):
     duration: Optional[datetime.timedelta] = None
     """The duration of the item as an alternative to an explicit end date/time."""
 
-    exdate: list[Union[datetime.datetime, datetime.date]] = Field(default_factory=list)
-    """Defines the list of exceptions for recurring todo item.
-
-    The exception dates are used in computing the recurrence set. The recurrence set is
-    the complete set of recurrence instances for a calendar component (based on rrule, rdate,
-    exdate). The recurrence set is generated by gathering the rrule and rdate properties
-    then excluding any times specified by exdate.
-    """
-
     geo: Optional[Geo] = None
     """Specifies a latitude and longitude global position for the activity."""
 
@@ -212,29 +203,9 @@ def start_datetime(self) -> datetime.datetime | None:
     @property
     def computed_duration(self) -> datetime.timedelta | None:
         """Return the event duration."""
-        if self.due is None:
+        if self.due is None or self.dtstart is None:
             return None
-        return self.due - self.start
-
-    def __lt__(self, other: Any) -> bool:
-        if not isinstance(other, Todo):
-            return NotImplemented
-        return self.due < other.due
-
-    def __gt__(self, other: Any) -> bool:
-        if not isinstance(other, Todo):
-            return NotImplemented
-        return self.due > other.due
-
-    def __le__(self, other: Any) -> bool:
-        if not isinstance(other, Todo):
-            return NotImplemented
-        return self.due <= other.due
-
-    def __ge__(self, other: Any) -> bool:
-        if not isinstance(other, Todo):
-            return NotImplemented
-        return self.due >= other.due
+        return self.due - self.dtstart
 
     @property
     def recurring(self) -> bool:
@@ -258,7 +229,7 @@ def as_rrule(self) -> Iterable[datetime.datetime | datetime.date] | None:
 
         This is only valid for events where `recurring` is True.
         """
-        if not self.rrule and not self.rdate:
+        if not (self.rrule or self.rdate) or not self.start:
             return None
         return RulesetIterable(
             self.start,