diff --git a/doc/howtos/backend/exercise-calendar b/doc/howtos/backend/exercise-calendar
index 8f3b59658e8..1ef0cbe2ca7 100644
--- a/doc/howtos/backend/exercise-calendar
+++ b/doc/howtos/backend/exercise-calendar
@@ -21,7 +21,7 @@ diff --git a/openacademy/models.py b/openacademy/models.py
  
      @api.depends('seats', 'attendee_ids')
      def _taken_seats(self):
-@@ -83,6 +86,30 @@ class Session(models.Model):
+@@ -83,6 +86,27 @@ class Session(models.Model):
                  },
              }
  
@@ -34,9 +34,8 @@ diff --git a/openacademy/models.py b/openacademy/models.py
 +
 +            # Add duration to start_date, but: Monday + 5 days = Saturday, so
 +            # subtract one second to get on Friday instead
-+            start = fields.Datetime.from_string(r.start_date)
 +            duration = timedelta(days=r.duration, seconds=-1)
-+            r.end_date = start + duration
++            r.end_date = r.start_date + duration
 +
 +    def _set_end_date(self):
 +        for r in self:
@@ -45,9 +44,7 @@ diff --git a/openacademy/models.py b/openacademy/models.py
 +
 +            # Compute the difference between dates, but: Friday - Monday = 4 days,
 +            # so add one day to get 5 days instead
-+            start_date = fields.Datetime.from_string(r.start_date)
-+            end_date = fields.Datetime.from_string(r.end_date)
-+            r.duration = (end_date - start_date).days + 1
++            r.duration = (r.end_date - r.start_date).days + 1
 +
      @api.constrains('instructor_id', 'attendee_ids')
      def _check_instructor_not_in_attendees(self):
diff --git a/doc/howtos/backend/exercise-gantt b/doc/howtos/backend/exercise-gantt
index 1e4aedd9624..54d3cb5b37f 100644
--- a/doc/howtos/backend/exercise-gantt
+++ b/doc/howtos/backend/exercise-gantt
@@ -15,9 +15,9 @@ diff --git a/openacademy/models.py b/openacademy/models.py
      @api.depends('seats', 'attendee_ids')
      def _taken_seats(self):
          for r in self:
-@@ -110,6 +113,15 @@ class Session(models.Model):
-             end_date = fields.Datetime.from_string(r.end_date)
-             r.duration = (end_date - start_date).days + 1
+@@ -107,6 +110,15 @@ class Session(models.Model):
+             # so add one day to get 5 days instead
+             r.duration = (r.end_date - r.start_date).days + 1
  
 +    @api.depends('duration')
 +    def _get_hours(self):
diff --git a/doc/howtos/backend/exercise-graph b/doc/howtos/backend/exercise-graph
index 39d06f74944..8fb2a46dc1f 100644
--- a/doc/howtos/backend/exercise-graph
+++ b/doc/howtos/backend/exercise-graph
@@ -15,7 +15,7 @@ diff --git a/openacademy/models.py b/openacademy/models.py
      @api.depends('seats', 'attendee_ids')
      def _taken_seats(self):
          for r in self:
-@@ -122,6 +125,11 @@ class Session(models.Model):
+@@ -119,6 +122,11 @@ class Session(models.Model):
          for r in self:
              r.duration = r.hours / 24
  
diff --git a/doc/howtos/backend/exercise-translations b/doc/howtos/backend/exercise-translations
index 08c013783cf..3cd57d2fd33 100644
--- a/doc/howtos/backend/exercise-translations
+++ b/doc/howtos/backend/exercise-translations
@@ -48,7 +48,7 @@ diff --git a/openacademy/models.py b/openacademy/models.py
                  },
              }
  
-@@ -135,4 +135,4 @@ class Session(models.Model):
+@@ -132,4 +132,4 @@ class Session(models.Model):
      def _check_instructor_not_in_attendees(self):
          for r in self:
              if r.instructor_id and r.instructor_id in r.attendee_ids: