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 4d27c86583..6aa19feadf 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/sequence/edit_task_time.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/sequence/edit_task_time.py
@@ -40,11 +40,11 @@ class Usecase:
             del self.settings["attributes"]["ScheduleFinish"]
 
         duration_type = self.settings["attributes"].get("DurationType", self.settings["task_time"].DurationType)
-        if "ScheduleFinish" in self.settings["attributes"]:
+        if self.settings["attributes"].get("ScheduleFinish", None):
             self.settings["attributes"]["ScheduleFinish"] = ifcopenshell.util.sequence.get_soonest_working_day(
                 self.settings["attributes"]["ScheduleFinish"], duration_type, self.calendar
             )
-        if "ScheduleStart" in self.settings["attributes"]:
+        if self.settings["attributes"].get("ScheduleStart", None):
             self.settings["attributes"]["ScheduleStart"] = ifcopenshell.util.sequence.get_soonest_working_day(
                 self.settings["attributes"]["ScheduleStart"], duration_type, self.calendar
             )
@@ -63,7 +63,7 @@ class Usecase:
             and self.settings["task_time"].ScheduleStart
         ):
             self.calculate_finish()
-        elif "ScheduleStart" in self.settings["attributes"].keys() and self.settings["task_time"].ScheduleDuration:
+        elif self.settings["attributes"].get("ScheduleStart", None) and self.settings["task_time"].ScheduleDuration:
             self.calculate_finish()
         elif self.settings["attributes"].get("ScheduleFinish", None) and self.settings["task_time"].ScheduleStart:
             self.calculate_duration()
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 4ba66461fb..4055b0194d 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
@@ -88,6 +88,26 @@ class TestEditTaskTime(test.bootstrap.IFC4):
         assert task_time.ScheduleFinish is None
         assert task_time.ScheduleDuration is None
 
+    def test_editing_just_a_start_date_with_no_duration_or_finish_but_with_a_calendar(self):
+        ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcProject")
+        calendar = ifcopenshell.api.run("sequence.add_work_calendar", self.file)
+        task = self.file.createIfcTask()
+        ifcopenshell.api.run("control.assign_control", self.file, relating_control=calendar, related_object=task)
+        task_time = ifcopenshell.api.run("sequence.add_task_time", self.file, task=task)
+        ifcopenshell.api.run(
+            "sequence.edit_task_time",
+            self.file,
+            task_time=task_time,
+            attributes={
+                "ScheduleDuration": None,
+                "ScheduleStart": datetime.datetime(2000, 1, 1),
+                "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(