Had a chat with ticketing department, and they also did confirm that it's valid from first segment, not date of issuance.

I wonder how serious the "not valid after" column of tickets are treated by airline. sometimes, travel agents put wrong dates on this column for no apparant reason, and in fact mine says "not valid after 2nd of Dec". The Ticketing department can easily confirm the actual validity of my ticket, but it's when I'm talking to under-knowledged MPC staffs, these little notes makes it extremely hard for me to convince that my ticket is valid through Febuary.