**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
for payslip in self:
            copied_payslip = payslip.copy({'credit_note': True, 'name': _('Refund: ') + payslip.name})
            copied_payslip.compute_sheet()
            copied_payslip.action_payslip_done()
        formview_ref = self.env.ref('hr_payroll.view_hr_payslip_form', False)
        treeview_ref = self.env.ref('hr_payroll.view_hr_payslip_tree', False)
        return {
            'name': ("Refund Payslip"),
            'view_mode': 'tree, form',
            'view_id': False,
            'view_type': 'form',
            'res_model': 'hr.payslip',
            'type': 'ir.actions.act_window',
            'target': 'current',
            'domain': "[('id', 'in', %s)]" % copied_payslip.ids,
            'views': [(treeview_ref and treeview_ref.id or False, 'tree'), (formview_ref and formview_ref.id or False, 'form')],
            'context': {}
        }
    @api.multi
    def check_done(self):
        return True
    @api.multi
    def unlink(self):
        if any(self.filtered(lambda payslip: payslip.state not in ('draft', 'cancel'))):
            raise UserError(_('You cannot delete a payslip which is not draft or cancelled!'))
        return super(HrPayslip, self).unlink()
    
    @api.model
    def get_contract(self, employee, date_from, date_to):