If a cells date isnt entered using the same date system then Excel wont recognize it as a true date. In this case you can simply use the built-in System File Checker utility. If you are not sure about the spelling or the spelling is ok you can resolve the error by having Excel insert the formula for you.
In the Task Manager Warning dialog box click Yes.
Excel uses your computers date system. Save my name email and website in this browser for the next time I comment. Download and run the Stellar excel file repair on your device version. Now select the damaged Excel file to be fixed.