I have this 'on_change' function:

remaining_days = fields.Integer(string='Remaining Days', store=True)

@api.onchange('current_date', 'payment_due_date', 'remaining_days')
def calculate_remaining_days(self):
    if self.current_date and self.payment_due_date:
        start_date = self.current_date
        end_date = self.payment_due_date
        days = (end_date - start_date).days
        self.remaining_days = days

Upon saving the record, the value of remaing_days is automatically converted to zero (0)

What could be possibly wrong?