How to use Django ORM to function on a field

This question is a follow-up to this one.

I'm running a Django application on top of a MySQL (actually MariaDB) database.

My Django Model looks like this:

from django.db import models
from django.db.models import Count, Sum

class myModel(models.Model):
    my_string = models.CharField(max_length=32,)
    my_date = models.DateTimeField()

    @staticmethod
    def get_stats():            
        logger.info(myModel.objects.values('my_string').annotate(
                count=Count("my_string"), 
                sum1=Sum('my_date'),
                sum2=Sum(# WHAT GOES HERE?? #),
            )
        )

When I call get_stats, it gives me the count and the sum1. However, for sum2, I want the sum of the following Database expression for each matching row: my_date + 0 (this converts it to a true integer value).

What should I put in the expression above to get that sum returned in sum2?