Hello Omar,

You can calculate with new field for duration like how many month,week,year etc.. instead of date field for duration.

You can do it like following :

 duration = fields.Char('Duration')
    duration_unit = fields.Selection([('month','Month'),('week','Week'),('day','Day'),('year','Year')])

Your calculation should be like following :

from dateutil.relativedelta import relativedelta

class MyMarketProduct(models.Model):
    _name = 'market.products'

    def _compute_expiry(self):
        
	if self.duration_unit=='month':
	    expiry_date=self.production_date + relativedelta(months = int(self.duration))
        elif self.duration_unit=='week':
	    expiry_date=self.production_date + relativedelta(weeks = int(self.duration))
        elif self.duration_unit=='day'                     
            expiry_date=self.production_date + relativedelta(days = int(self.duration))
        elif self.duration_unit=='year'                     
            expiry_date=self.production_date + relativedelta(years = int(self.duration)) 
                                   
	self.expiration_date=expiry_date

Thanks,