**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
import time
from datetime import datetime
from datetime import time as datetime_time
from dateutil import relativedelta
import babel
from odoo import api, fields, models, tools, _
from odoo.addons import decimal_precision as dp
from odoo.exceptions import UserError, ValidationError
class HrPayslip(models.Model):
    _name = 'hr.payslip'
    _description = 'Pay Slip'
    struct_id = fields.Many2one('hr.payroll.structure', string='Structure',
        readonly=True, states={'draft': [('readonly', False)]},
        help='Defines the rules that have to be applied to this payslip, accordingly '
             'to the contract chosen. If you let empty the field contract, this field isn\'t '
             'mandatory anymore and thus the rules applied will be all the rules set on the '
             'structure of all contracts of the employee valid for the chosen period')
    name = fields.Char(string='Payslip Name', readonly=True,
        states={'draft': [('readonly', False)]})
    number = fields.Char(string='Reference', readonly=True, copy=False,
        states={'draft': [('readonly', False)]})
    employee_id = fields.Many2one('hr.employee', string='Employee', required=True, readonly=True,
        states={'draft': [('readonly', False)]})
    date_from = fields.Date(string='Date From', readonly=True, required=True,
        default=time.strftime('%Y-%m-01'), states={'draft': [('readonly', False)]})
    date_to = fields.Date(string='Date To', readonly=True, required=True,
        default=str(datetime.now() + relativedelta.relativedelta(months=+1, day=1, days=-1))[:10],
        states={'draft': [('readonly', False)]})