diff --git a/kfet/statistic.py b/kfet/statistic.py
index 2abb4fd8..1db30d73 100644
--- a/kfet/statistic.py
+++ b/kfet/statistic.py
@@ -79,12 +79,6 @@ class StatScale(object):
             label_fmt = self.label_fmt
         return [begin.strftime(label_fmt) for begin, end in self]
 
-    @classmethod
-    def get_chunk_start(cls, dt):
-        dt_kfet = to_kfet_day(dt)
-        start = dt_kfet - cls.offset_to_chunk_start(dt_kfet)
-        return start
-
 
 class DayStatScale(StatScale):
     name = 'day'
@@ -102,8 +96,10 @@ class WeekStatScale(StatScale):
     label_fmt = 'Semaine %W'
 
     @classmethod
-    def offset_to_chunk_start(cls, dt):
-        return timezone.timedelta(days=dt.weekday())
+    def get_chunk_start(cls, dt):
+        dt_kfet = to_kfet_day(dt)
+        offset = timezone.timedelta(days=dt_kfet.weekday())
+        return dt_kfet - offset
 
 
 class MonthStatScale(StatScale):