时间间隔提示无法做 - 运算是什么原因？

从Excel表格中获取的时间数据，想要做时间差计算，但是无法成功，

报错提示如下：

Traceback (most recent call last):
File "/Applications/影刀.app/Contents/Resources/app/node_modules/xbot-engine/lib/lib/python3.7/runpy.py", line 193, in _run_module_as_main
"__main__", mod_spec)
File "/Applications/影刀.app/Contents/Resources/app/node_modules/xbot-engine/lib/lib/python3.7/runpy.py", line 85, in _run_code
exec(code, run_globals)
File "/Applications/影刀.app/Contents/Resources/app/node_modules/xbot-engine/lib/lib/python3.7/site-packages/xbot_interpreter.py", line 87, in <module>
main()
File "/Applications/影刀.app/Contents/Resources/app/node_modules/xbot-engine/lib/lib/python3.7/site-packages/xbot_interpreter.py", line 47, in main
try:
File "/Users/daisydo/Library/Application Support/Shadowbot/users/80446125931540481/apps/be7ea723-9832-4ed7-a616-3d262b858d8c/xbot_robot/process1.py", line 20, in main
datetime_difference = xbot_visual.datetime.difference(begin_datetime=lambda: current_datetime, end_datetime=lambda: due_date, unit="day", _block=("数据读取与发送", 11, "获取时间间隔"))
File "<string>", line 89, in wrapper
File "<string>", line 84, in wrapper
File "<string>", line 218, in difference

TypeError: unsupported operand type(s) for -: 'objc.pyobjc_unicode' and 'datetime.datetime'