You would need a model to store it in. If you have a custom User model or a user profile model, you could add a field to either of them to store it. It’s a DateTimeField that you are saving.
Ok, so you would be fine with UTC dates.
You can subtract two datetime fields, the result is a timedelta object. If that object represents more than 24 hours, then you update your model with the current time.
See the docs at Signals | Django documentation | Django to get started.