You can claim bonus AC on the TDM if the thread in was started after you were accepted, but you can't claim bonus AC on threads you began before you were accepted! For the purposes of judging when a thread was started, we look at the date on the initial comment responding to a toplevel prompt, rather than the toplevel's date itself.

That means if someone tagged into your toplevel, or you tagged someone else's toplevel, those threads can be counted towards bonus AC as long as it happened after you were accepted. Does that make sense? Let us know if you need more clarification!