**Impacted versions:**
11.0

**Steps to reproduce:**
In runbot odoo/odoo or odoo/enterprise

- In Preferences change the timezone to America/Mexico_city

- Create a contract for a user with this values: **state**: running, **End Date**: NULL, **Start Date**: year before (for example), and **working schedule**: Standard 40 Hours/week
![image](https://user-images.githubusercontent.com/19410285/35942162-2640db56-0c2b-11e8-8714-b25413d62123.png)

- Edit the working schedule as the image below, and save.
![image](https://user-images.githubusercontent.com/19410285/35942368-c846cf82-0c2b-11e8-9a26-753ded2509d1.png)

- Create a payslip for a user with the edited contract.

- Change the date_from (in period) to a saturday day
![image](https://user-images.githubusercontent.com/19410285/35942575-83626d08-0c2c-11e8-8023-11675d29c3ea.png)


**Current behavior:**
Take this error
![image](https://user-images.githubusercontent.com/19410285/35942631-ad048268-0c2c-11e8-9147-c3f2ff5770fe.png)


**Expected behavior:**
compute the number of days without error


**Video/Screenshot link (optional):**
https://youtu.be/fQWTg7LXD2g

addons/hr_payroll/models/hr_payslip.py
if to_date is None:
                    to_date = fields.Date.today()
                self.env.cr.execute("""SELECT sum(case when hp.credit_note = False then (pl.total) else (-pl.total) end)
                            FROM hr_payslip as hp, hr_payslip_line as pl
                            WHERE hp.employee_id = %s AND hp.state = 'done'
                            AND hp.date_from >= %s AND hp.date_to <= %s AND hp.id = pl.slip_id AND pl.code = %s""",
                            (self.employee_id, from_date, to_date, code))
                res = self.env.cr.fetchone()
                return res and res[0] or 0.0
        
        result_dict = {}
        rules_dict = {}
        worked_days_dict = {}
        inputs_dict = {}
        blacklist = []
        payslip = self.env['hr.payslip'].browse(payslip_id)
        for worked_days_line in payslip.worked_days_line_ids:
            worked_days_dict[worked_days_line.code] = worked_days_line
        for input_line in payslip.input_line_ids:
            inputs_dict[input_line.code] = input_line
        categories = BrowsableObject(payslip.employee_id.id, {}, self.env)
        inputs = InputLine(payslip.employee_id.id, inputs_dict, self.env)
        worked_days = WorkedDays(payslip.employee_id.id, worked_days_dict, self.env)
        payslips = Payslips(payslip.employee_id.id, payslip, self.env)
        rules = BrowsableObject(payslip.employee_id.id, rules_dict, self.env)
        baselocaldict = {'categories': categories, 'rules': rules, 'payslip': payslips, 'worked_days': worked_days, 'inputs': inputs}
        
        contracts = self.env['hr.contract'].browse(contract_ids)
        if len(contracts) == 1 and payslip.struct_id:
            structure_ids = list(set(payslip.struct_id._get_parent_structure().ids))