Django: How to automatically change a field's value at the time mentioned in the same object?

I am working on a django project for racing event in which a table in the database has three fields.

1)Boolean field to know whether race is active or not

2)Race start time

3)Race end time

While creating an object of it,the start_time and end_time are specified. How to change the value of boolean field to True when the race starts and
to False when it ends? How to schedule these activities?