am developing a module in odoo 10 that requires me to get time difference 

i have created 3 fields in my model of which the time difference field is to be filled automatically after computation

opened_field = fields.Datetime(string="DateTime Opened", required=False,)
closed_field = fields.Datetime(string="DateTime Closed", required=False,)
time_diff = fields.Float(string="Time Difference",  required=False )

and below is my function

@api.onchange('opened_field', 'closed_field','time_diff')
def calculate_date(self):
    if self.opened_field and self.closed_field:
        t1=datetime.strptime(str(self.opened_field),'%Y-%m-%d %H:%M:%S')
        t2=datetime.strptime(str(self.closed_field),'%%Y-%m-%d %H:%M:%S')
        t3=t2-t1
        self.time_diff=str(t3.days)

on running the code am getting this error 

Traceback (most recent call last):
  File "/home/rottal/Desktop/odoo-dev/odoo/odoo/http.py", line 642, in _handle_exception
    return super(JsonRequest, self)._handle_exception(exception)
  File "/home/rottal/Desktop/odoo-dev/odoo/odoo/http.py", line 684, in dispatch
    result = self._call_function(**self.params)
  File "/home/rottal/Desktop/odoo-dev/odoo/odoo/http.py", line 334, in _call_function
    return checked_call(self.db, *args, **kwargs)
  File "/home/rottal/Desktop/odoo-dev/odoo/odoo/service/model.py", line 101, in wrapper
    return f(dbname, *args, **kwargs)
  File "/home/rottal/Desktop/odoo-dev/odoo/odoo/http.py", line 327, in checked_call
    result = self.endpoint(*a, **kw)
  File "/home/rottal/Desktop/odoo-dev/odoo/odoo/http.py", line 942, in __call__
    return self.method(*args, **kw)
  File "/home/rottal/Desktop/odoo-dev/odoo/odoo/http.py", line 507, in response_wrap
    response = f(*args, **kw)
  File "/home/rottal/Desktop/odoo-dev/odoo/addons/web/controllers/main.py", line 892, in call_kw
    return self._call_kw(model, method, args, kwargs)
  File "/home/rottal/Desktop/odoo-dev/odoo/addons/web/controllers/main.py", line 884, in _call_kw
    return call_kw(request.env[model], method, args, kwargs)
  File "/home/rottal/Desktop/odoo-dev/odoo/odoo/api.py", line 689, in call_kw
    return call_kw_multi(method, model, args, kwargs)
  File "/home/rottal/Desktop/odoo-dev/odoo/odoo/api.py", line 680, in call_kw_multi
    result = method(recs, *args, **kwargs)
  File "/home/rottal/Desktop/odoo-dev/odoo/odoo/models.py", line 5518, in onchange
    record._onchange_eval(name, field_onchange[name], result)
  File "/home/rottal/Desktop/odoo-dev/odoo/odoo/models.py", line 5416, in _onchange_eval
    method_res = method(self)
  File "/home/rottal/Desktop/odoo-dev/todo_app/new_connections/models/connection.py", line 68, in calculate_date
    t1=datetime.strptime(str(self.opened_field),'%Y-%m-%d %H:%M:%S')
AttributeError: 'module' object has no attribute 'strptime'

 what might be the problem kindly assist