**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

attendances = {
                'name': _("Normal Working Days paid at 100%"),
                'sequence': 1,
                'code': 'WORK100',
                'number_of_days': work_data['days'],
                'number_of_hours': work_data['hours'],
                'contract_id': contract.id,
            }
            res.append(attendances)
            res.extend(leaves.values())
        return res
    @api.model
    def get_inputs(self, contracts, date_from, date_to):
        res = []
        structure_ids = contracts.get_all_structures()
        rule_ids = self.env['hr.payroll.structure'].browse(structure_ids).get_all_rules()
        sorted_rule_ids = [id for id, sequence in sorted(rule_ids, key=lambda x:x[1])]
        inputs = self.env['hr.salary.rule'].browse(sorted_rule_ids).mapped('input_ids')
        for contract in contracts:
            for input in inputs:
                input_data = {
                    'name': input.name,
                    'code': input.code,
                    'contract_id': contract.id,
                }
                res += [input_data]
        return res
    @api.model
    def _get_payslip_lines(self, contract_ids, payslip_id):
        def _sum_salary_rule_category(localdict, category, amount):