from odoo.fields import Date
from dateutil.relativedelta import relativedelta

@api.onchange('duration')
def compute_status(self):
if self.duration: 
    for record in self.subscription:
        record.write({'subscription_months_calc': record.duration + record.subscription_months_calc})
       
        if record.subscription_registration_date:
            expiry_date = Date.from_string(record.subscription_registration_date) + relativedelta(
                months=record.subscription_months_calc)
            record.write({'subscription_expiry_date': Date.to_string(expiry_date)})