I have joined as an AT on 23/09/02. HM noted in my COSA form that my service confirmation date is 05/12/03. Is it possible?
I think, you have joined in your first service on 23/09/02 as an assistant teacher. So your confirmation date will be 23/09/2004. Most probably it (05/12/03) may be the approval date of your appointment by the D.I. of Schools.
You asked to your H.M. to correct it.