diff --git a/docs/settings.rst b/docs/settings.rst
index 9dd96f40..a78fd2af 100644
--- a/docs/settings.rst
+++ b/docs/settings.rst
@@ -298,6 +298,26 @@ Time Tracking Options
 
   **Default:** ``HELPDESK_FOLLOWUP_TIME_SPENT_AUTO = False``
 
+- **HELPDESK_FOLLOWUP_TIME_SPENT_OPENING_HOURS** If defined, calculates follow-up 'time_spent' according to open hours.
+
+  **Default:** ``HELPDESK_FOLLOWUP_TIME_SPENT_OPENING_HOURS = {}``
+  
+  If HELPDESK_FOLLOWUP_TIME_SPENT_AUTO is ``True``, you may set open hours to remove off hours from 'time_spent'::
+
+        HELPDESK_FOLLOWUP_TIME_SPENT_OPENING_HOURS = {
+            "monday": (8.5, 19),
+            "tuesday": (8.5, 19),
+            "wednesday": (8.5, 19),
+            "thursday": (8.5, 19),
+            "friday": (8.5, 19),
+            "saturday": (0, 0),
+            "sunday": (0, 0),
+        }
+  
+  Valid hour values must be set between 0 and 23.9999.
+  In this example 8.5 is interpreted as 8:30AM, saturdays and sundays don't count.
+
+
 Staff Ticket Creation Settings
 ------------------------------
 
diff --git a/helpdesk/lib.py b/helpdesk/lib.py
index ac0d46bc..10d53736 100644
--- a/helpdesk/lib.py
+++ b/helpdesk/lib.py
@@ -194,3 +194,40 @@ def convert_value(value):
         return value.strftime(CUSTOMFIELD_TIME_FORMAT)
     else:
         return value
+
+
+def daily_time_spent_calculation(earliest, latest, open_hours):
+    """Returns timedelta for a single day time interval according to open hours."""
+
+    # avoid rendering day in different locale
+    weekday = ('monday', 'tuesday', 'wednesday', 'thursday',
+                'friday', 'saturday', 'sunday')[earliest.weekday()]
+    
+    # enforce correct settings
+    MIDNIGHT = 23.9999
+    start, end = open_hours.get(weekday, (0, MIDNIGHT))
+    if not 0 <= start <= end <= MIDNIGHT:
+        start, end = 0, MIDNIGHT
+    
+    # transform decimals to minutes
+    start_hour, start_minute, start_second = int(start), int(start % 1 * 60), int(start * 60 % 1 * 60)
+    end_hour, end_minute, end_second = int(end), int(end % 1 * 60), int(end * 60 % 1 * 60)
+
+    # translate time for delta calculation
+    earliest_f = earliest.hour + earliest.minute / 60
+    latest_f = latest.hour + latest.minute / 60
+
+    if earliest_f < start:
+        earliest = earliest.replace(hour=start_hour, minute=start_minute, second=start_second)
+    elif earliest_f >= end:
+        earliest = earliest.replace(hour=end_hour, minute=end_minute, second=end_second)
+    
+    if latest_f < start:
+        latest = latest.replace(hour=start_hour, minute=start_minute, second=start_second)
+    elif latest_f >= end:
+        latest = latest.replace(hour=end_hour, minute=end_minute, second=end_second)
+    
+    day_delta = latest - earliest
+
+    # returns up to 86399 seconds
+    return day_delta.seconds
\ No newline at end of file
diff --git a/helpdesk/models.py b/helpdesk/models.py
index a258a315..9cd4de81 100644
--- a/helpdesk/models.py
+++ b/helpdesk/models.py
@@ -8,7 +8,7 @@ models.py - Model (and hence database) definitions. This is the core of the
 """
 
 
-from .lib import convert_value
+from .lib import convert_value, daily_time_spent_calculation
 from .templated_email import send_templated_mail
 from .validators import validate_file_extension
 from .webhooks import send_new_ticket_webhook
@@ -1008,7 +1008,7 @@ class FollowUp(models.Model):
                 latest_time = latest_fup.date
             except ObjectDoesNotExist:
                 latest_time = t.created
-            self.time_spent = now - latest_time
+            self.time_spent = self.time_spent_calculation(latest_time, now)
         t.modified = now
         t.save()
         super(FollowUp, self).save(*args, **kwargs)
@@ -1020,6 +1020,32 @@ class FollowUp(models.Model):
     def time_spent_formated(self):
         return format_time_spent(self.time_spent)
 
+    def time_spent_calculation(self, earliest, latest):
+        "Returns timedelta according to rules settings."
+
+        time_spent_seconds = 0
+        open_hours = helpdesk_settings.FOLLOWUP_TIME_SPENT_OPENING_HOURS
+
+        # split time interval by days
+        days = latest.toordinal() - earliest.toordinal()
+        if days:
+            for day in range(days + 1):
+                if day == 0:
+                    start_day_time = earliest
+                    end_day_time = earliest.replace(hour=23, minute=59, second=59)
+                elif day == days:
+                    start_day_time = latest.replace(hour=0, minute=0, second=0)
+                    end_day_time = latest
+                else:
+                    middle_day_time = earliest + timedelta(days=day)
+                    start_day_time = middle_day_time.replace(hour=0, minute=0, second=0)
+                    end_day_time = middle_day_time.replace(hour=23, minute=59, second=59)
+                time_spent_seconds += daily_time_spent_calculation(start_day_time, end_day_time, open_hours)
+        # handle same day
+        else:
+            time_spent_seconds += daily_time_spent_calculation(earliest, latest, open_hours)
+
+        return datetime.timedelta(seconds=time_spent_seconds)
 
 class TicketChange(models.Model):
     """
diff --git a/helpdesk/settings.py b/helpdesk/settings.py
index 220bc424..96eac1ee 100644
--- a/helpdesk/settings.py
+++ b/helpdesk/settings.py
@@ -161,6 +161,11 @@ FOLLOWUP_TIME_SPENT_AUTO = getattr(settings,
                                    'HELPDESK_FOLLOWUP_TIME_SPENT_AUTO',
                                    False)
 
+# Calculate time_spent according to open hours
+FOLLOWUP_TIME_SPENT_OPENING_HOURS = getattr(settings,
+                                   'HELPDESK_FOLLOWUP_TIME_SPENT_OPENING_HOURS',
+                                   {})
+
 ############################
 # options for public pages #
 ############################