diff --git a/inginious/frontend/pages/course_admin/statistics.py b/inginious/frontend/pages/course_admin/statistics.py
index 032bee3f8..75906da70 100644
--- a/inginious/frontend/pages/course_admin/statistics.py
+++ b/inginious/frontend/pages/course_admin/statistics.py
@@ -95,7 +95,7 @@ def _graph_stats(self, daterange, filter, limit):
             cur += increment
 
         for entry in stats_graph:
-            c = datetime(entry["_id"]["year"], entry["_id"]["month"], entry["_id"]["day"], 0 if method == "day" else entry["_id"]["hour"])
+            c = datetime(entry["_id"]["year"], entry["_id"]["month"], entry["_id"]["day"], 0 if method == "day" else entry["_id"]["hour"], tzinfo=timezone.utc)
             all_submissions[c] += entry["submissions"]
             valid_submissions[c] += entry["validSubmissions"]