**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

if not values.get('resource_id'):
            resource = self.env['resource.resource'].create({
                'name': values.get(self._rec_name)
            })
            values['resource_id'] = resource.id
        return super(ResourceMixin, self).create(values)
    @api.multi
    def copy_data(self, default=None):
        if default is None:
            default = {}
        resource = self.resource_id.copy()
        default['resource_id'] = resource.id
        default['company_id'] = resource.company_id.id
        default['resource_calendar_id'] = resource.calendar_id.id
        return super(ResourceMixin, self).copy_data(default)
    def get_work_days_count(self, from_datetime, to_datetime, calendar=None):