**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

help="Indicates this payslip has a refund of another")
    payslip_run_id = fields.Many2one('hr.payslip.run', string='Payslip Batches', readonly=True,
        copy=False, states={'draft': [('readonly', False)]})
    payslip_count = fields.Integer(compute='_compute_payslip_count', string="Payslip Computation Details")
    @api.multi
    def _compute_details_by_salary_rule_category(self):
        for payslip in self:
            payslip.details_by_salary_rule_category = payslip.mapped('line_ids').filtered(lambda line: line.category_id)
    @api.multi
    def _compute_payslip_count(self):
        for payslip in self:
            payslip.payslip_count = len(payslip.line_ids)
    @api.constrains('date_from', 'date_to')
    def _check_dates(self):
        if any(self.filtered(lambda payslip: payslip.date_from > payslip.date_to)):
            raise ValidationError(_("Payslip 'Date From' must be before 'Date To'."))
    @api.multi
    def action_payslip_draft(self):
        return self.write({'state': 'draft'})
    @api.multi
    def action_payslip_done(self):
        self.compute_sheet()
        return self.write({'state': 'done'})
    @api.multi
    def action_payslip_cancel(self):
        if self.filtered(lambda slip: slip.state == 'done'):
            raise UserError(_("Cannot cancel a payslip that is done."))
        return self.write({'state': 'cancel'})
    @api.multi
    def refund_sheet(self):