I don't know that there's a worksheet function to provide that information,
but you can create your own!  Called a 'User Defined Function', or UDF.

Public Function GetModifiedDate() As Date
   Dim fs, f
   Application.Volatile
   Set fs = CreateObject("Scripting.FileSystemObject")
   Set f = fs.GetFile(ThisWorkbook.FullName)
   GetModifiedDate = f.DateLastModified
   Set f = Nothing
   Set fs = Nothing
End Function

To use this, open the workbook and press [Alt]+[F11] and then choose Insert

code area and close the window.
Next, in the cell where you want the last modifed date to show up put this
formula:
=GetModifiedDate()

NOTE: in order for this to work, the workbook must have been saved to disk
at least once.

ANOTHER NOTE: this may not automatically update.  The Application.Volatile
statement tells it to update whenever any new calculation takes place on that
worksheet, but if no new calculations take place, then it won't update
either.  You may have to press [F9] to get the latest date for the file.

Probably a much better way!

Public Function GetModifiedDate() As Date
 Application.Volatile
 GetModifiedDate = _
  ThisWorkbook.BuiltinDocumentProperties("Last Save Time")
End Function

do same as before - copy it into the workbook (delete previous function
w/same name if you already copied it into the workbook), then set up the
formula in the cell where you want the data/time of last modification to show
up.

Try  the following code:

Public Function LastSaved(Optional FileName As String) As Variant
   If FileName = vbNullString Then
       If ThisWorkbook.Path = vbNullString Then
           LastSaved = CVErr(xlErrValue)
       Else
           LastSaved = FileDateTime(ThisWorkbook.FullName)
       End If
   Else
       If Dir(FileName, vbNormal) = vbNullString Then
           LastSaved = CVErr(xlErrValue)
       Else
           LastSaved = FileDateTime(FileName)
       End If
   End If
End Function

You can call it from a cell with =LastSaved()