Are you using QV12? If not then the new sorting syntax won't work. Look here: The sortable Aggr function is finally here!
I read the link you posted to me , if i am correct , they post is mentioned that new feature is support QS and ver 12.
Since now i am still using ver 9 , so your posted expression with the red high lighted word on sort month , i have tested it is not working in ver 9 , but work in QS.
So can i said that ver 9 i am unable make it work , and i need to look for alternative ?