**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
_name = 'hr.payslip.run'
    _description = 'Payslip Batches'
    name = fields.Char(required=True, readonly=True, states={'draft': [('readonly', False)]})
    slip_ids = fields.One2many('hr.payslip', 'payslip_run_id', string='Payslips', readonly=True,
        states={'draft': [('readonly', False)]})
    state = fields.Selection([
        ('draft', 'Draft'),
        ('close', 'Close'),
    ], string='Status', index=True, readonly=True, copy=False, default='draft')
    date_start = fields.Date(string='Date From', required=True, readonly=True,
        states={'draft': [('readonly', False)]}, default=time.strftime('%Y-%m-01'))
    date_end = fields.Date(string='Date To', required=True, readonly=True,
        states={'draft': [('readonly', False)]},
        default=str(datetime.now() + relativedelta.relativedelta(months=+1, day=1, days=-1))[:10])
    credit_note = fields.Boolean(string='Credit Note', readonly=True,
        states={'draft': [('readonly', False)]},
        help="If its checked, indicates that all payslips generated from here are refund payslips.")
    @api.multi
    def draft_payslip_run(self):
        return self.write({'state': 'draft'})
    @api.multi
    def close_payslip_run(self):
        return self.write({'state': 'close'})