It can be done easily in odoo 14:

start_date = fields.Date(string="Start date")
end_date = fields.Date(string="End date")
date_diff = fields.Char("Date difference")

@api.constrains('start_date', 'end_date', 'date_diff')
def _date_difference(self):
    for record in self:
        if record.start_date > record.end_date:
            raise ValidationError('Start date should not greater than end date.')
        else:
            record.date_diff = (record.end_date - record.start_date).days

finally it works 

thanks a lot Avinash you save my life

this is the entier final code :

in .py:

...

class EcoStage(models.Model):

_name ='eco.stage'

start_date = fields.Date()

end_date = fields.Date()

duree_stage = fields.Integer(string="duree stage", compute='difference_date', store=True)

@api.multi

@api.depends('  start_date ',' end_date ')

def difference_date(self):

fmt = '%Y-%m-%d'

start_date = self.start_date

end_date = self.end_date

d1 = datetime.strptime(start_date, fmt)

d2 = datetime.strptime(end_date, fmt)

if d2 >d1 :

self.  duree_stage = (d2 - d1).days

else :

raise ValueError('end date should be superior than start day')

...

in xml :

<fields name="duree_stage "/>