My requirement is 

hire_date = fields.Date; 

calculate_start = fields.Date;

If hire_date  is after calculate_start ,the date of calculate_start should be equal hire_date

hr_employee_base.py

from odoo.tools import format_time
from datetime import date, datetime, timedelta, time
from time import time
import datetime
import time
import calendar

##### the hire_date;resign_date;calcucate_start  must be fields.Date

hire_date = fields.Date(string="Hire Date")

calculate_start = fields.Date(string="Start Date", store=True, compute='_get_calculate_date')

@api.onchange('hire_date')
def _get_calcuate_date(self):
    firstday = datetime.datetime.strptime(self.hire_date, "%Y-%m-%d")
    secondday = datetime.datetime.strptime(self.calculate_start, "%Y-%m-%d")
    if firstday > secondday:
          self.calculate_start = self.hire_date

File "/opt/bitnami/apps/odoo/lib/odoo-13.0.post20191110-py3.7.egg/odoo/addons/hr/models/hr_employee_base.py", line 110, in _get_calcuate_date
    firstday = datetime.datetime.strptime(self.hire_date, "%Y-%m-%d")
TypeError: strptime() argument 1 must be str, not datetime.date