Openerp 7 based on this requirement Sale order lines Remaining days Calculate Start Date minus Today at all times. Functional Field and on_change function used:-

def _remaining_days(self, cr, uid, ids, field_name, arg, context=None):
            res = {}
            if not ids:
                return {}
            for val in self.browse(cr, uid, ids, context=context):
                result = datetime.datetime.strptime(val.start_date, '%Y-%m-%d') - datetime.datetime.strptime(current_date, '%Y-%m-%d')
                res[val.id] = result.days
            return res

    'remaining_days': fields.function(_remaining_days, method=True, string='Remaining days', type='integer'),


    def onchange_holddays(self, cr, uid, ids, start_date, context=None):
        result = {}  
        context = context or {}
        if release_date:
            current_date = time.strftime('%Y-%m-%d')
            remaining = datetime.datetime.strptime(release_date, '%Y-%m-%d') - datetime.datetime.strptime(current_date, '%Y-%m-%d')
            result['remaining_days'] = remaining.days                
        return {'value': result}

In order lines Remaining days Values not updated automatically. once save the main sale order record then only lines Remaining days value updated. How to shows the remaining days values in list view before saving the record.