diff --git a/addons/mass_mailing/mass_mailing.py b/addons/mass_mailing/mass_mailing.py
index 516e68db3d9..e7583d8e739 100644
--- a/addons/mass_mailing/mass_mailing.py
+++ b/addons/mass_mailing/mass_mailing.py
@@ -168,8 +168,10 @@ class MassMailing(osv.Model):
                            'tooltip': (date_begin + relativedelta.relativedelta(days=i)).strftime('%d %B %Y'),
                            } for i in range(0, self._period_number)]
         group_obj = obj.read_group(cr, uid, domain, read_fields, groupby_field, context=context)
+        field_col_info = obj._all_columns.get(groupby_field.split(':')[0])
+        pattern = tools.DEFAULT_SERVER_DATE_FORMAT if field_col_info.column._type == 'date' else tools.DEFAULT_SERVER_DATETIME_FORMAT
         for group in group_obj:
-            group_begin_date = datetime.strptime(group['__domain'][0][2], tools.DEFAULT_SERVER_DATE_FORMAT).date()
+            group_begin_date = datetime.strptime(group['__domain'][0][2], pattern).date()
             timedelta = relativedelta.relativedelta(group_begin_date, date_begin)
             section_result[timedelta.days] = {'value': group.get(value_field, 0), 'tooltip': group.get(groupby_field)}
         return section_result