I want to have the date entered in cell O3 when Completed is chosen

I want to have the date entered in cell O3 when Completed is chosen from a drop down list in M3, the date needs to stay as the date completed was entered
I need this to be repeated from cell O3 M3 all the way down the work sheet.

I Have tried to write up a code but i am a novice

Private Sub Worksheet_Change(ByVal Target As Range)

If Not Intersect(Target, Range("M3")) Is Nothing Then_

Target.Offset(-1, 0).Value = Now

End Sub

Any Suggestions
I have attached worksheet
See April Sheet