Yes, it is possible to change the creation/modified dates in Excel. However, it is not recommended to do so as it can lead to legal issues and can be considered as tampering with documents.

To change the creation/modified dates in Excel, you can follow these steps:

1. Open the Excel file that you want to modify the creation/modified dates for.
2. Click on the "File" tab in the ribbon.
3. Click on "Info" in the left-hand menu.
4. Click on "Properties" in the right-hand menu.
5. Click on "Advanced Properties" at the bottom of the window.
6. In the "Summary" tab, you will see the "Created" and "Modified" dates. You can change these dates to any date you want.
7. Click "OK" to save the changes.

However, if you suspect that someone has tampered with the creation/modified dates of an Excel file, there are ways to find out. You can check the file's metadata to see if the dates have been modified. You can also check the file's properties to see if the dates match the file's content. If you have concerns about the authenticity of a document, it is best to consult with a legal professional.