diff --git a/src/ifcopenshell-python/ifcopenshell/api/sequence/calculate_task_duration.py b/src/ifcopenshell-python/ifcopenshell/api/sequence/calculate_task_duration.py
index 4a3f5b1413..2c53705e74 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/sequence/calculate_task_duration.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/sequence/calculate_task_duration.py
@@ -47,11 +47,11 @@ class Usecase:
         work_day_duration = ifcopenshell.util.date.ifc2datetime(psets["Pset_WorkControlCommon"]["WorkDayDuration"])
         return work_day_duration.seconds
 
-    def get_work_schedule(self):
-        for rel in self.settings["task"].HasAssignments or []:
+    def get_work_schedule(self, task):
+        for rel in task.HasAssignments or []:
             if rel.is_a("IfcRelAssignsToControl") and rel.RelatingControl.is_a("IfcWorkSchedule"):
                 return rel.RelatingControl
-        for rel in self.settings["task"].Nests or []:
+        for rel in task.Nests or []:
             return self.get_work_schedule(rel.RelatingObject)
 
     def calculate_duration_in_days(self, resource):
diff --git a/src/ifcopenshell-python/test/api/sequence/test_calculate_task_duration.py b/src/ifcopenshell-python/test/api/sequence/test_calculate_task_duration.py
index 7fe64f7fe1..7718af4025 100644
--- a/src/ifcopenshell-python/test/api/sequence/test_calculate_task_duration.py
+++ b/src/ifcopenshell-python/test/api/sequence/test_calculate_task_duration.py
@@ -24,3 +24,47 @@ class TestCalculateTaskDuration(test.bootstrap.IFC4):
         ifcopenshell.api.run("sequence.assign_process", self.file, relating_process=task, related_object=resource)
         ifcopenshell.api.run("sequence.calculate_task_duration", self.file, task=task)
         assert task.TaskTime.ScheduleDuration == "P4D"
+
+    def test_calculating_a_task_duration_without_a_work_schedule_defining_workday_duration(self):
+        ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcProject")
+        task = ifcopenshell.api.run("sequence.add_task", self.file)
+        resource = ifcopenshell.api.run("resource.add_resource", self.file, ifc_class="IfcLaborResource")
+        resource_time = ifcopenshell.api.run("resource.add_resource_time", self.file, resource=resource)
+        resource_time.ScheduleWork = "P2D"
+        ifcopenshell.api.run("sequence.assign_process", self.file, relating_process=task, related_object=resource)
+        ifcopenshell.api.run("sequence.calculate_task_duration", self.file, task=task)
+        assert task.TaskTime.ScheduleDuration == "P2D"
+
+    def test_calculating_a_task_duration_with_a_custom_workday_duration(self):
+        ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcProject")
+        schedule = ifcopenshell.api.run("sequence.add_work_schedule", self.file)
+        pset = ifcopenshell.api.run("pset.add_pset", self.file, product=schedule, name="Pset_WorkControlCommon")
+        ifcopenshell.api.run("pset.edit_pset", self.file, pset=pset, properties={"WorkDayDuration": "PT2H"})
+        task = ifcopenshell.api.run("sequence.add_task", self.file, work_schedule=schedule)
+        resource = ifcopenshell.api.run("resource.add_resource", self.file, ifc_class="IfcLaborResource")
+        resource_time = ifcopenshell.api.run("resource.add_resource_time", self.file, resource=resource)
+        resource_time.ScheduleWork = "PT48H"
+        ifcopenshell.api.run("sequence.assign_process", self.file, relating_process=task, related_object=resource)
+        ifcopenshell.api.run("sequence.calculate_task_duration", self.file, task=task)
+        assert task.TaskTime.ScheduleDuration == "P24D"
+
+    def test_failing_to_calculate_if_no_schedule_work_usage(self):
+        ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcProject")
+        schedule = ifcopenshell.api.run("sequence.add_work_schedule", self.file)
+        task = ifcopenshell.api.run("sequence.add_task", self.file, work_schedule=schedule)
+        resource = ifcopenshell.api.run("resource.add_resource", self.file, ifc_class="IfcLaborResource")
+        ifcopenshell.api.run("sequence.assign_process", self.file, relating_process=task, related_object=resource)
+        ifcopenshell.api.run("sequence.calculate_task_duration", self.file, task=task)
+        assert task.TaskTime is None
+
+    def test_calculating_a_nested_task_duration_based_on_a_labour_resource_with_work_hours(self):
+        ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcProject")
+        schedule = ifcopenshell.api.run("sequence.add_work_schedule", self.file)
+        task = ifcopenshell.api.run("sequence.add_task", self.file, work_schedule=schedule)
+        subtask = ifcopenshell.api.run("sequence.add_task", self.file, parent_task=task)
+        resource = ifcopenshell.api.run("resource.add_resource", self.file, ifc_class="IfcLaborResource")
+        resource_time = ifcopenshell.api.run("resource.add_resource_time", self.file, resource=resource)
+        resource_time.ScheduleWork = "PT48H"
+        ifcopenshell.api.run("sequence.assign_process", self.file, relating_process=subtask, related_object=resource)
+        ifcopenshell.api.run("sequence.calculate_task_duration", self.file, task=subtask)
+        assert subtask.TaskTime.ScheduleDuration == "P6D"