**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/resource/models/resource_mixin.py
from datetime import timedelta
from odoo import api, fields, models
from odoo.tools import float_utils
class ResourceMixin(models.AbstractModel):
    _name = "resource.mixin"
    _description = 'Resource Mixin'
    resource_id = fields.Many2one(
        'resource.resource', 'Resource',
        auto_join=True, index=True, ondelete='restrict', required=True)
    company_id = fields.Many2one(
        'res.company', 'Company',
        default=lambda self: self.env['res.company']._company_default_get(),
        index=True, related='resource_id.company_id', store=True)
    resource_calendar_id = fields.Many2one(
        'resource.calendar', 'Working Hours',
        default=lambda self: self.env['res.company']._company_default_get().resource_calendar_id,
        index=True, related='resource_id.calendar_id')
    @api.model
    def create(self, values):