def get_inputs(self, contract_ids, date_from, date_to):
    """This Compute the other inputs to employee payslip.
                       """
    res = super(SalaryRuleInput, self).get_inputs(contract_ids, date_from, date_to)
    contract_obj = self.env['hr.contract']
    emp_id = contract_obj.browse(contract_ids[0].id).employee_id
    adv_salary = self.env['salary.advance'].search([('employee_id', '=', emp_id.id)])
    for adv_obj in adv_salary:
        current_date = datetime.strptime(date_from, '%Y-%m-%d').date().month
        date = adv_obj.date
        existing_date = datetime.strptime(date, '%Y-%m-%d').date().month
        if current_date == existing_date:
            state = adv_obj.state
            amount = adv_obj.advance
            for result in res:
                if state == 'approve' and amount != 0 and result.get('code') == 'SAR':
                    result['amount'] = amount
    return res

In this method odoo 12 gives me an error in the line 

current_date = datetime.strptime(date_from, '%Y-%m-%d').date().month

It said that first argument must be a string. Can you tell me how to fix it and why it is working in version 11?

Thanks in advance