make sure redis command filters dates properly

also write the tests that i should have written a while ago for this

app/commands.py

@@ -504,7 +504,12 @@ def populate_redis_template_usage(service_id, day):
     Recalculate and replace the stats in redis for a day.
     To be used if redis data is lost for some reason.
     """
+    # the day variable is set to midnight of that day
     assert current_app.config['REDIS_ENABLED']
+
+    start_time = get_london_midnight_in_utc(day)
+    end_time = get_london_midnight_in_utc(day + timedelta(days=1))
+
     usage = {
         str(row.template_id): row.count
         for row in db.session.query(
@@ -512,6 +517,8 @@ def populate_redis_template_usage(service_id, day):
             func.count().label('count')
         ).filter(
             Notification.service_id == service_id,
+            Notification.created_at >= start_time,
+            Notification.created_at < end_time
         ).group_by(
             Notification.template_id
         )