Do not set default init date in Tummy Time card declaration.

This would cause the date to default to the date the instance starts, not the date the method is called from a template.

dashboard/templatetags/dashboard.py

@@ -2,7 +2,7 @@
 from __future__ import unicode_literals
 
 from django import template
-from django.utils import formats, timezone
+from django.utils import timezone
 
 from core.models import DiaperChange, Feeding, Sleep, TummyTime
 
@@ -75,9 +75,11 @@ def card_tummytime_last(child):
 
 
 @register.inclusion_tag('cards/tummytime_day.html')
-def card_tummytime_day(child, date=timezone.localtime().date()):
+def card_tummytime_day(child, date=None):
     """Tummy time over the course of `date`.
     """
+    if not date:
+        date = timezone.localtime().date()
     instances = TummyTime.objects.filter(
         child=child, end__day=date.day).order_by('-end')
     stats = {