diff --git a/src/blenderbim/blenderbim/bim/module/sequence/helper.py b/src/blenderbim/blenderbim/bim/module/sequence/helper.py
index 8046e09174..1fd685fd57 100644
--- a/src/blenderbim/blenderbim/bim/module/sequence/helper.py
+++ b/src/blenderbim/blenderbim/bim/module/sequence/helper.py
@@ -1,8 +1,84 @@
+import math
 import isodate
+import datetime
 from dateutil import parser
 from ifcopenshell.api.sequence.data import Data
 
 
+def count_working_days(start, finish, calendar):
+    result = 0
+    current_date = datetime.date(start.year, start.month, start.day)
+    finish_date = datetime.date(finish.year, finish.month, finish.day)
+    while current_date <= finish_date:
+        if is_working_day(current_date, calendar):
+            result += 1
+        current_date += datetime.timedelta(days=1)
+    return result
+
+
+def is_working_day(day, calendar):
+    is_working_day = False
+    for work_time_id in calendar["WorkingTimes"] or []:
+        if is_work_time_applicable_to_day(Data.work_times[work_time_id], day):
+            is_working_day = True
+            break
+    if not is_working_day:
+        return is_working_day
+    for work_time_id in calendar["ExceptionTimes"] or []:
+        if is_work_time_applicable_to_day(Data.work_times[work_time_id], day):
+            is_working_day = False
+            break
+    return is_working_day
+
+
+def is_work_time_applicable_to_day(work_time, day):
+    if work_time["Start"] and work_time["Start"] > day:
+        return False
+
+    if work_time["Finish"] and work_time["Finish"] < day:
+        return False
+
+    if not work_time["RecurrencePattern"]:
+        return True
+
+    recurrence = Data.recurrence_patterns[work_time["RecurrencePattern"]]
+
+    if recurrence["RecurrenceType"] == "DAILY":
+        if not recurrence["Interval"] and not recurrence["Occurrences"]:
+            return True
+        if not work_time["Start"]:
+            return False
+        return False  # TODO
+    elif recurrence["RecurrenceType"] == "WEEKLY":
+        if not recurrence["Interval"] and not recurrence["Occurrences"]:
+            return (day.weekday() + 1) in recurrence["WeekdayComponent"]
+        if not work_time["Start"]:
+            return False
+        return False  # TODO
+    elif recurrence["RecurrenceType"] == "MONTHLY_BY_DAY_OF_MONTH":
+        if not recurrence["Interval"] and not recurrence["Occurrences"]:
+            return day.day in recurrence["DayComponent"]
+        return False  # TODO
+    elif recurrence["RecurrenceType"] == "MONTHLY_BY_POSITION":
+        if not recurrence["Interval"] and not recurrence["Occurrences"]:
+            return (day.weekday() + 1) in recurrence["WeekdayComponent"] and math.floor(day.day / 7) + 1 == recurrence[
+                "Position"
+            ]
+        return False  # TODO
+    elif recurrence["RecurrenceType"] == "YEARLY_BY_DAY_OF_MONTH":
+        if not recurrence["Interval"] and not recurrence["Occurrences"]:
+            return day.month in recurrence["MonthComponent"] and day.day in recurrence["DayComponent"]
+        return False  # TODO
+    elif recurrence["RecurrenceType"] == "YEARLY_BY_POSITION":
+        if not recurrence["Interval"] and not recurrence["Occurrences"]:
+            return (
+                day.month in recurrence["MonthComponent"]
+                and (day.weekday() + 1) in recurrence["WeekdayComponent"]
+                and math.floor(day.day / 7) + 1 == recurrence["Position"]
+            )
+        return False  # TODO
+
+
 def derive_date(ifc_definition_id, attribute_name, date=None, is_earliest=False, is_latest=False):
     task = Data.tasks[ifc_definition_id]
     if task["TaskTime"]:
@@ -31,6 +107,7 @@ def parse_datetime(value):
         except:
             return None
 
+
 def parse_duration(value):
     try:
         return isodate.parse_duration(value)
diff --git a/src/blenderbim/blenderbim/bim/module/sequence/operator.py b/src/blenderbim/blenderbim/bim/module/sequence/operator.py
index 73d649c05d..4d4c5314fb 100644
--- a/src/blenderbim/blenderbim/bim/module/sequence/operator.py
+++ b/src/blenderbim/blenderbim/bim/module/sequence/operator.py
@@ -303,9 +303,9 @@ class LoadTaskProperties(bpy.types.Operator):
                 derived_finish = helper.derive_date(item.ifc_definition_id, "ScheduleFinish", is_latest=True)
                 item.derived_start = self.canonicalise_time(derived_start) if derived_start else ""
                 item.derived_finish = self.canonicalise_time(derived_finish) if derived_finish else ""
-                if derived_start and derived_finish:
-                    derived_duration = ifcopenshell.util.date.timedelta2duration(derived_finish - derived_start)
-                    item.derived_duration = isodate.duration_isoformat(derived_duration)
+                if derived_start and derived_finish and calendar:
+                    derived_duration = helper.count_working_days(derived_start, derived_finish, calendar)
+                    item.derived_duration = f"P{derived_duration}D"
                 item.start = "-"
                 item.finish = "-"
                 item.duration = "-"