You can add this capability to your workbook, it's not a built in function. Here's the UDF code for the new function:
Code:
Function LastSaved() As Date
Application.Volatile
LastSaved = ThisWorkbook.BuiltinDocumentProperties(12)
End Function
How to install the User Defined Function:
1. Open up your workbook
2. Get into VB Editor
(Press Alt+F11)
3. Insert a new module
(Insert > Module)
4. Copy and Paste in your code (given above)
5. Get out of VBA
(Press Alt+Q)
6. Save your sheet
The function is installed and ready to use.
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In a cell, enter this formula:
=LASTSAVED()
The cell will not return a value properly until the sheet has been saved. Format the cell to display the date/time to your liking.