You could use an UDF like this too:
<font face=Courier New>
<SPAN style="color:#00007F">Function</SPAN> GetSavedDate(<SPAN style="color:#00007F">Optional</SPAN> Rng <SPAN style="color:#00007F">As</SPAN> Range) <SPAN style="color:#00007F">As</SPAN> <SPAN style="color:#00007F">Date</SPAN>
<SPAN style="color:#00007F">On</SPAN> <SPAN style="color:#00007F">Error</SPAN> <SPAN style="color:#00007F">Resume</SPAN> <SPAN style="color:#00007F">Next</SPAN>
<SPAN style="color:#00007F">If</SPAN> Rng <SPAN style="color:#00007F">Is</SPAN> <SPAN style="color:#00007F">Nothing</SPAN> <SPAN style="color:#00007F">Then</SPAN>
GetSavedDate = Application.Caller.Parent.Parent.BuiltinDocumentProperties("Last Save Time").Value
<SPAN style="color:#00007F">Else</SPAN>
GetSavedDate = Rng.Parent.Parent.BuiltinDocumentProperties("Last Save Time").Value
<SPAN style="color:#00007F">End</SPAN> <SPAN style="color:#00007F">If</SPAN>
<SPAN style="color:#00007F">End</SPAN> <SPAN style="color:#00007F">Function</SPAN>
</FONT>
Using it like
=GETSAVEDDATE()
to get the info for the current workbook, or like
=GETSAVEDDATE(A1)
where A1 refers to a cell in the workbook that you want to know the info (I *think*, didn't test it, that requires the workbook to be open)