There is a formula that will return the current date/time. It is:
=Now()
However, note that is not static, so it will continue to change.
If you want to capture the date/time that it was opened, and "freeze it" in time, you would need to use VBA.
There are literally thousands of threads on this, called "Date Stamp" or "DateTime Stamp", though most of them will be using the "Worksheet_Change" event procedure, which runs when a cell is changed, whereas you will probably need to use a "Workbook_Open" event procedure, which runs when a workbook is opened.
If you are open to VBA and need help doing that, please provide the sheet name and cell that you want this value to go into.