Django GROUP BY strftime date format

I would like to do a SUM on rows in a database and group by date.

I am trying to run this SQL query using Django aggregates and annotations:

select strftime('%m/%d/%Y', time_stamp) as the_date, sum(numbers_data)
    from my_model
    group by the_date;

I tried the following:

data = My_Model.objects.values("strftime('%m/%d/%Y',
           time_stamp)").annotate(Sum("numbers_data")).order_by()

but it seems like you can only use column names in the values() function; it doesn't like the use of strftime().

How should I go about this?