how to calculate difference between two dates ? | Odoo

from datetime import datetime

@api.onchange('demo_time_start', 'demo_time_end', 'demo_day')def calculate_date(self):    if self.demo_time_start and self.demo_time_end:        d1 = datetime.strptime(str(self.demo_time_start), '%Y-%m-%d')        d2 = datetime.strptime(str(self.demo_time_end), '%Y-%m-%d')        d3 = d2 - d1
        self.demo_day = str(d3.days)