**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
"""
        @param employee: recordset of employee
        @param date_from: date field
        @param date_to: date field
        @return: returns the ids of all the contracts for the given employee that need to be considered for the given dates
        """
        
        clause_1 = ['&', ('date_end', '<=', date_to), ('date_end', '>=', date_from)]
        
        clause_2 = ['&', ('date_start', '<=', date_to), ('date_start', '>=', date_from)]
        
        clause_3 = ['&', ('date_start', '<=', date_from), '|', ('date_end', '=', False), ('date_end', '>=', date_to)]
        clause_final = [('employee_id', '=', employee.id), ('state', '=', 'open'), '|', '|'] + clause_1 + clause_2 + clause_3
        return self.env['hr.contract'].search(clause_final).ids
    @api.multi
    def compute_sheet(self):
        for payslip in self:
            number = payslip.number or self.env['ir.sequence'].next_by_code('salary.slip')
            
            payslip.line_ids.unlink()
            
            
            contract_ids = payslip.contract_id.ids or \
                self.get_contract(payslip.employee_id, payslip.date_from, payslip.date_to)
            lines = [(0, 0, line) for line in self._get_payslip_lines(contract_ids, payslip.id)]
            payslip.write({'line_ids': lines, 'number': number})
        return True
    @api.model