it probably doesn't matter because it's such a small block of code, but for future reference there's no need to declare the MyCol2 variable as you can use Target directly. Also for future reference, when you declare variables don't forget to declare the type. If you end up making a big application with lots of code that will speed things up.
So if you did need to declare MyCol2 you would do it like so:
Dim MyCol2 as string
Anyway, here's the fastest most efficient way to do what you're doing:
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Column = 13 Then 'it's the 13th column, in other words, "M"
Application.EnableEvents = False
Target.Offset(0, 10) = Date
Target.Offset(0, 11) = Time
Application.EnableEvents = True