diff --git a/proposals/migrations/0022_auto_20161019_1141.py b/proposals/migrations/0022_auto_20161019_1141.py
new file mode 100644
index 0000000..ad7b9a6
--- /dev/null
+++ b/proposals/migrations/0022_auto_20161019_1141.py
@@ -0,0 +1,40 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.10.1 on 2016-10-19 11:41
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+def migrate_durations(apps, schema_editor):
+    db_alias = schema_editor.connection.alias
+    Event = apps.get_model('proposals', 'Event')
+    for event in Event.objects.all():
+        if event.duration < 0:
+            event.duration = 0
+            event.save()
+    Talk = apps.get_model('proposals', 'Talk')
+    for talk in Talk.objects.all():
+        if talk.duration < 0:
+            talk.duration = 0
+            talk.save()
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('proposals', '0021_auto_20161016_2121'),
+    ]
+
+    operations = [
+        migrations.RunPython(migrate_durations),
+        migrations.AlterField(
+            model_name='event',
+            name='duration',
+            field=models.PositiveIntegerField(default=0, verbose_name='Default duration (min)'),
+        ),
+        migrations.AlterField(
+            model_name='talk',
+            name='duration',
+            field=models.PositiveIntegerField(default=0, verbose_name='Duration (min)'),
+        ),
+    ]
diff --git a/proposals/models.py b/proposals/models.py
index a21a6d4..58b9b98 100644
--- a/proposals/models.py
+++ b/proposals/models.py
@@ -53,7 +53,7 @@ class Track(PonyConfModel):
         unique_together = ('site', 'name')
 
     def estimated_duration(self):
-        return sum([talk.estimated_duration() for talk in self.talk_set.all()])
+        return sum([talk.estimated_duration for talk in self.talk_set.all()])
 
     def __str__(self):
         return self.name
@@ -87,7 +87,7 @@ class Event(models.Model):
 
     site = models.ForeignKey(Site, on_delete=models.CASCADE)
     name = models.CharField(max_length=64)
-    duration = models.IntegerField(default=0, verbose_name=_('Default duration (min)'))
+    duration = models.PositiveIntegerField(default=0, verbose_name=_('Default duration (min)'))
     color = RGBColorField(default='#ffffff', verbose_name=_("Color on program"))
 
     class Meta:
@@ -117,7 +117,7 @@ class Talk(PonyConfModel):
     event = models.ForeignKey(Event, verbose_name=_('Intervention kind'))
     accepted = models.NullBooleanField(default=None)
     start_date = models.DateTimeField(null=True, blank=True, default=None)
-    duration = models.IntegerField(default=0, verbose_name=_('Duration (min)'))
+    duration = models.PositiveIntegerField(default=0, verbose_name=_('Duration (min)'))
     room = models.ForeignKey(Room, blank=True, null=True, default=None)
 
     class Meta:
@@ -133,6 +133,7 @@ class Talk(PonyConfModel):
         else:
             return ', '.join(speakers[:-1]) + ' & ' + str(speakers[-1])
 
+    @property
     def estimated_duration(self):
         return self.duration or self.event.duration
 
@@ -163,10 +164,12 @@ class Talk(PonyConfModel):
         else:
             return 0
 
-
     @property
     def end_date(self):
-        return self.start_date + timedelta(minutes=self.duration)
+        if self.estimated_duration:
+            return self.start_date + timedelta(minutes=self.estimated_duration)
+        else:
+            return None
 
     class Meta:
         ordering = ('event__id',)
diff --git a/proposals/templates/proposals/talk_detail.html b/proposals/templates/proposals/talk_detail.html
index efefe7c..bdd2e40 100644
--- a/proposals/templates/proposals/talk_detail.html
+++ b/proposals/templates/proposals/talk_detail.html
@@ -36,8 +36,7 @@
     <dt>Horaire</dt>
       <dd>{% if talk.start_date %}
           <span class="date">{{ talk.start_date|date:"l d b" }}</span>,
-          <span class="time">{{ talk.start_date|date:"H:i" }} &ndash;
-            {{ talk.end_date|date:"H:i" }}</span>
+          <span class="time">{{ talk.start_date|date:"H:i" }} &ndash; {% if talk.end_date %}{{ talk.end_date|date:"H:i" }}{% else %}?{% endif %}</span>
           {% else %}<em>{% trans "not defined" %}</em>
           {% endif %}
       </dd>