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

Thank you for your reply
I need it to go down the whole collum and i need it to enter the date in the corresponding row
E.g when completed is selected in M3, cell O3 shows the date that completed was selected
this goes down the sheet so that everytime cell M? has completed the corresponding cell row in O shows the date that completed was selected
Cheers