diff --git a/src/blenderbim/blenderbim/bim/module/sequence/prop.py b/src/blenderbim/blenderbim/bim/module/sequence/prop.py
index 7d21dc1d04..47e7e0146b 100644
--- a/src/blenderbim/blenderbim/bim/module/sequence/prop.py
+++ b/src/blenderbim/blenderbim/bim/module/sequence/prop.py
@@ -104,6 +104,7 @@ def updateTaskTimeDateTime(self, context, startfinish):
         **{"task_time": task_time, "attributes": {startfinish_key: startfinish_datetime}},
     )
     Data.load(IfcStore.get_file())
+    bpy.ops.bim.load_task_properties(task=props.active_task_id)
     setattr(self, startfinish, canonicalise_time(startfinish_datetime))
 
 
@@ -127,6 +128,7 @@ def updateTaskduration(self, context):
     if props.active_task_id == self.ifc_definition_id:
         attribute = props.task_attributes.get("Duration")
         attribute.string_value = self.duration
+    bpy.ops.bim.load_task_properties(task=props.active_task_id)
 
 
 def updateVisualisationStart(self, context):
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 ed5b5b369d..e4c3f8118a 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/sequence/edit_task_time.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/sequence/edit_task_time.py
@@ -30,24 +30,48 @@ class Usecase:
             and self.settings["task_time"].ScheduleDuration
             and self.settings["task_time"].ScheduleStart
         ):
-            start = ifcopenshell.util.date.ifc2datetime(self.settings["task_time"].ScheduleStart)
-            current_date = datetime.date(start.year, start.month, start.day)
-            duration = ifcopenshell.util.date.ifc2datetime(self.settings["task_time"].ScheduleDuration).days
+            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:
+            self.calculate_duration()
 
-            task = [e for e in self.file.get_inverse(self.settings["task_time"]) if e.is_a("IfcTask")]
-            if not task:
-                return
-            else:
-                task = task[0]
+    def calculate_finish(self):
+        start = ifcopenshell.util.date.ifc2datetime(self.settings["task_time"].ScheduleStart)
+        current_date = datetime.date(start.year, start.month, start.day)
+        duration = ifcopenshell.util.date.ifc2datetime(self.settings["task_time"].ScheduleDuration).days
 
-            calendar = ifcopenshell.util.sequence.derive_calendar(task)
+        calendar = self.get_calendar()
 
-            while duration >= 0:
-                if self.settings["task_time"].DurationType == "ELAPSEDTIME" or not calendar:
-                    duration -= 1
-                elif ifcopenshell.util.sequence.is_working_day(current_date, calendar):
-                    duration -= 1
-                current_date += datetime.timedelta(days=1)
+        while duration >= 0:
+            if self.settings["task_time"].DurationType == "ELAPSEDTIME" or not calendar:
+                duration -= 1
+            elif ifcopenshell.util.sequence.is_working_day(current_date, calendar):
+                duration -= 1
+            current_date += datetime.timedelta(days=1)
 
-            current_date -= datetime.timedelta(days=1)
-            self.settings["task_time"].ScheduleFinish = ifcopenshell.util.date.datetime2ifc(current_date, "IfcDateTime")
+        current_date -= datetime.timedelta(days=1)
+        self.settings["task_time"].ScheduleFinish = ifcopenshell.util.date.datetime2ifc(current_date, "IfcDateTime")
+
+    def calculate_duration(self):
+        start = ifcopenshell.util.date.ifc2datetime(self.settings["task_time"].ScheduleStart)
+        finish = ifcopenshell.util.date.ifc2datetime(self.settings["task_time"].ScheduleFinish)
+        current_date = datetime.date(start.year, start.month, start.day)
+        finish_date = datetime.date(finish.year, finish.month, finish.day)
+        calendar = self.get_calendar()
+        duration = datetime.timedelta()
+        while current_date < finish_date:
+            if self.settings["task_time"].DurationType == "ELAPSEDTIME" or not calendar:
+                duration += datetime.timedelta(days=1)
+            elif ifcopenshell.util.sequence.is_working_day(current_date, calendar):
+                duration += datetime.timedelta(days=1)
+            current_date += datetime.timedelta(days=1)
+        self.settings["task_time"].ScheduleDuration = ifcopenshell.util.date.datetime2ifc(duration, "IfcDuration")
+
+    def get_calendar(self):
+        task = [e for e in self.file.get_inverse(self.settings["task_time"]) if e.is_a("IfcTask")]
+        if not task:
+            return
+        else:
+            task = task[0]
+        return ifcopenshell.util.sequence.derive_calendar(task)