diff --git a/src/ifcopenshell-python/ifcopenshell/api/sequence/edit_task_time.py b/src/ifcopenshell-python/ifcopenshell/api/sequence/edit_task_time.py
index 722ea46f45..4d27c86583 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/sequence/edit_task_time.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/sequence/edit_task_time.py
@@ -65,7 +65,7 @@ class Usecase:
             self.calculate_finish()
         elif "ScheduleStart" in self.settings["attributes"].keys() and self.settings["task_time"].ScheduleDuration:
             self.calculate_finish()
-        elif "ScheduleFinish" in self.settings["attributes"].keys() and self.settings["task_time"].ScheduleStart:
+        elif self.settings["attributes"].get("ScheduleFinish", None) and self.settings["task_time"].ScheduleStart:
             self.calculate_duration()
 
         if self.settings["task_time"].ScheduleDuration and (
diff --git a/src/ifcopenshell-python/test/api/sequence/test_edit_task_time.py b/src/ifcopenshell-python/test/api/sequence/test_edit_task_time.py
index 25b945fdc7..4ba66461fb 100644
--- a/src/ifcopenshell-python/test/api/sequence/test_edit_task_time.py
+++ b/src/ifcopenshell-python/test/api/sequence/test_edit_task_time.py
@@ -72,6 +72,22 @@ class TestEditTaskTime(test.bootstrap.IFC4):
         assert task_time.RemainingTime == "P1D"
         assert task_time.Completion == 0.5
 
+    def test_editing_just_a_start_date_with_no_duration_or_finish(self):
+        task_time = ifcopenshell.api.run("sequence.add_task_time", self.file, task=self.file.createIfcTask())
+        ifcopenshell.api.run(
+            "sequence.edit_task_time",
+            self.file,
+            task_time=task_time,
+            attributes={
+                "ScheduleDuration": None,
+                "ScheduleStart": "2000-01-01T00:00:00",
+                "ScheduleFinish": None,
+            },
+        )
+        assert task_time.ScheduleStart == "2000-01-01T00:00:00"
+        assert task_time.ScheduleFinish is None
+        assert task_time.ScheduleDuration is None
+
     def test_schedule_finish_dates_are_auto_calculated_if_possible(self):
         task_time = ifcopenshell.api.run("sequence.add_task_time", self.file, task=self.file.createIfcTask())
         ifcopenshell.api.run(