Okay, let's go

.
If you want it just for one workbook, activatev that workbook.
First go into the VBE - Alt-F11
Then go to the Insert>Module menu, a code pane window will popup
Copy my code into that window
Close the VBE (x in the corner is OK)
Now test it in the workbook, and save the workbook.
If you want it as a general purpose function, then we'll put it in
Personal.xls
Go to menu Tools>Macro>Record New Macro
In the 'Store macro in' dropdown, select Personal.xls
Give the macro any old name (we will remove it later)
Stop the macro (Tools>Macro>Stop Recording)
Go into the VBE - Alt-F11
Select Personal.xls fro the list of workbooks in the project explorer
Open all sections (the + sign)
In the Modules section, select Module1 and paste the code in.
Close the VBE.
Test it. You will now need to use =Personal.xls!DocProps("last save time")
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HTH
RP
(remove nothere from the email address if mailing direct)