You're correct that this will require VBA. Which means this probably should have been posted in the Programming forum. Oops.
Right-click the sheet tab and select VIEW CODE, then paste in this macro:
Option Explicit
Private Sub Worksheet_Change(ByVal Target As Range)
Dim cell As Range
For Each cell In Target
If Not Intersect(cell, Range("A:J")) Is Nothing Then
If cell <> "" Then
Application.EnableEvents = False
Range("L" & cell.Row) = Date
Application.EnableEvents = True
End If
End If
Next cell
End Sub
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