You could try setting both dates to midnight:
class ValidEntryManager(models.Manager):
def get_query_set(self):
today = datetime.today()
start_date = datetime(today.year, today.month, today.day)
end_date = datetime(today.year, today.month, today.day+1)
return super(ValidEntryManager, self).get_query_set().filter(timestamp__gte=start_date, timestamp__lte=end_date)