Date Time Stamp

I am not sure how to write a formula to give me the current date/time once a file is open and to remain that original time no matter how many times the file may be opened subsequently after that. I cannot use =today() as that will change every time I open the file.

The time and date would be in two different cells. I am not even sure how to do a timestamp at all either, even if it does change each time I open the file.

Thank you for any assistance with this at all.