@Adam **********
it appears from talking to someone who took that extension (not at Chaengwattana though) that it starts the day you apply for it. That is because it's issued for a different reason; not clause 2.18; marriage to a thai but clause 2.24 visiting thai family and they seem to start it the day you apply.
BUT
seeing as I am not married to a thai, nor do I have thai family I honestly have no
idea you need to slog on out to C/W yourself if you're wanting to get that extension and ask them ๐