I am not aware of a formula that can do that, but you can achieve this effect by using VBA.
Locate the Excel workbook icon just to the left of the File menu option, near the upper left corner of your screen. Right click on that icon, then left click on View Code, and paste this in:
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Private Sub Workbook_BeforeSave(ByVal SaveAsUI As Boolean, Cancel As Boolean)
Sheet1.[A1] = "File was last saved " & Now
End Sub
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This will place the information you want in cell A1 of Sheet1; modify for sheet and desired cell.
HTH