diff --git a/addons/hr_timesheet_sheet/hr_timesheet_sheet.py b/addons/hr_timesheet_sheet/hr_timesheet_sheet.py
index f8984fd4a28..876a871e488 100644
--- a/addons/hr_timesheet_sheet/hr_timesheet_sheet.py
+++ b/addons/hr_timesheet_sheet/hr_timesheet_sheet.py
@@ -218,7 +218,8 @@ class hr_timesheet_sheet(osv.osv):
             return []
         if isinstance(ids, (long, int)):
             ids = [ids]
-        return [(r['id'], _('Week ')+datetime.strptime(r['date_from'], '%Y-%m-%d').strftime('%U')) \
+        # week number according to ISO 8601 Calendar
+        return [(r['id'], _('Week ')+str(datetime.strptime(r['date_from'], '%Y-%m-%d').isocalendar()[1])) \
                 for r in self.read(cr, uid, ids, ['date_from'],
                     context=context, load='_classic_write')]