For Excel 2000 and higher you can use this
ActiveWorkbook.BuiltinDocumentProperties("Last Save Time")
Note :
This is not working correct in Excel 97
Read this from Tom Ogilvy
Note that this property is not maintained in xl97. However, if the workbook
is created/used in both xl97 and later versions, then when used in the
later versions and saved the property is maintained but when used in xl97
and saved, the property is not altered. The time it was last saved in the
later version will be retrieved if this property is called. If created and
used exclusively in xl97, calling this property will raise an error. You
can use the beforesave event to update this property in xl97 as a
workaround. Just test the version of excel and if xl97, update the
property.