This tutorial demonstrates how to find the date a file was last saved in Excel and Google Sheets.
Find the Last Saved Date
To find the last saved date of an Excel file, in the Ribbon, go to File > Info.
In the Info window, on the right side, you can find the Last Modified (in this example 6/5/2015). This is the date and time when the file was last saved.
To get details like Related Dates within a macro, see VBA Get File & Folder Properties.
Last Saved Date in Google Sheets
All Google Sheets changes are saved automatically on edit. You can see when the last edit of the file was to the right of the Menu toolbar.