There are a few different ways that you can go about noting when a workbook was changed in Excel. One way is to simply look at the "Date Modified" column in the workbook's properties. This will give you a general idea of when the workbook was last changed. However, this method will not give you any specific information on what was changed within the workbook.
If you need more specific information on what was changed within the workbook, you can use the "Track Changes" feature. This feature will keep track of all changes made to the workbook, including who made the changes and when they were made. To enable the "Track Changes" feature, go to the "Review" tab and click on "Track Changes". Once you have enabled this feature, any changes made to the workbook will be highlighted in red. You can then click on the "Changes" button in the "Review" tab to see a detailed list of all changes that have been made.
Another way to note when a workbook was changed is to use version control. Version control is a system that keeps track of all changes made to a file, including who made the changes and when they were made. Excel does not have a built-in version control system, but there are many third-party version control systems that you can use. One popular option is Microsoft Visual SourceSafe.
No matter which method you use, it is important to keep track of when your workbooks are changed. This information can be very useful if you ever need to go back and look at an older version of the workbook or if you need to troubleshoot an issue that arises.