diff --git a/openfisca_core/periods/date_unit.py b/openfisca_core/periods/date_unit.py
index a808b1941..13d93f8a2 100644
--- a/openfisca_core/periods/date_unit.py
+++ b/openfisca_core/periods/date_unit.py
@@ -98,6 +98,11 @@ class DateUnit(StrEnum, metaclass=DateUnitMeta):
 
     """
 
+    def __contains__(self, other: object) -> bool:
+        if isinstance(other, str):
+            return super().__contains__(other)
+        return NotImplemented
+
     WEEKDAY = "weekday"
     WEEK = "week"
     DAY = "day"
diff --git a/openfisca_core/periods/instant_.py b/openfisca_core/periods/instant_.py
index e68151c36..5eed50c45 100644
--- a/openfisca_core/periods/instant_.py
+++ b/openfisca_core/periods/instant_.py
@@ -90,6 +90,16 @@ def __str__(self) -> t.InstantStr:
 
         return instant_str
 
+    def __lt__(self, other: object) -> bool:
+        if isinstance(other, Instant):
+            return super().__lt__(other)
+        return NotImplemented
+
+    def __le__(self, other: object) -> bool:
+        if isinstance(other, Instant):
+            return super().__le__(other)
+        return NotImplemented
+
     @property
     def date(self) -> Date:
         instant_date = config.date_by_instant_cache.get(self)
diff --git a/openfisca_core/periods/period_.py b/openfisca_core/periods/period_.py
index 700652953..39884d900 100644
--- a/openfisca_core/periods/period_.py
+++ b/openfisca_core/periods/period_.py
@@ -389,7 +389,7 @@ def size_in_weekdays(self) -> int:
         if self.unit == DateUnit.YEAR:
             return self.size_in_weeks * 7
 
-        if self.unit in DateUnit.MONTH:
+        if DateUnit.MONTH in self.unit:
             last = self.start.offset(self.size, self.unit)
             if last is None:
                 raise NotImplementedError
@@ -685,10 +685,17 @@ def offset(self, offset: str | int, unit: t.DateUnit | None = None) -> t.Period:
 
         """
 
+        start: None | t.Instant = self[1].offset(
+            offset, self[0] if unit is None else unit
+        )
+
+        if start is None:
+            raise NotImplementedError
+
         return self.__class__(
             (
                 self[0],
-                self[1].offset(offset, self[0] if unit is None else unit),
+                start,
                 self[2],
             )
         )
diff --git a/openfisca_core/periods/types.py b/openfisca_core/periods/types.py
index f8136f5b6..26a6e096d 100644
--- a/openfisca_core/periods/types.py
+++ b/openfisca_core/periods/types.py
@@ -39,6 +39,12 @@ def day(self) -> int:
     def date(self) -> Date:
         ...
 
+    def __lt__(self, other: object, /) -> bool:
+        ...
+
+    def __le__(self, other: object, /) -> bool:
+        ...
+
     def offset(self, offset: str | int, unit: DateUnit) -> Instant | None:
         ...
 
diff --git a/openfisca_core/types.py b/openfisca_core/types.py
index fe9ea8dee..2c1e9945e 100644
--- a/openfisca_core/types.py
+++ b/openfisca_core/types.py
@@ -88,18 +88,14 @@ class ParameterNodeAtInstant(Protocol):
 # Periods
 
 
-class Container(Protocol[T_con]):
-    def __contains__(self, item: T_con, /) -> bool:
-        ...
-
-
 class Indexable(Protocol[T_cov]):
     def __getitem__(self, index: int, /) -> T_cov:
         ...
 
 
-class DateUnit(Container[str], Protocol):
-    ...
+class DateUnit(Protocol):
+    def __contains__(self, other: object, /) -> bool:
+        ...
 
 
 class Instant(Indexable[int], Iterable[int], Sized, Protocol):