diff --git a/basyx/aas/model/submodel.py b/basyx/aas/model/submodel.py
index 9f49a8aa..d7616753 100644
--- a/basyx/aas/model/submodel.py
+++ b/basyx/aas/model/submodel.py
@@ -16,7 +16,8 @@
 if TYPE_CHECKING:
     from . import aas
 
-from dateutil.relativedelta import relativedelta as Duration
+from dateutil.relativedelta import relativedelta
+Duration = relativedelta
 
 
 class SubmodelElement(base.Referable, base.Qualifiable, base.HasSemantics,
@@ -1339,11 +1340,11 @@ def last_update(self, last_update: Optional[datatypes.DateTime]) -> None:
         self._last_update: Optional[datatypes.DateTime] = last_update
 
     @property
-    def max_interval(self) -> Optional['Duration']:
+    def max_interval(self) -> Optional[Duration]:
         return self._max_interval
 
     @max_interval.setter
-    def max_interval(self, max_interval: Optional['Duration']) -> None:
+    def max_interval(self, max_interval: Optional[Duration]) -> None:
         if max_interval is not None and self.direction is base.Direction.INPUT:
             raise ValueError("max_interval is not applicable if direction = input!")
         self._max_interval: Optional['Duration'] = max_interval