Xiao Ming said: If I don't know, Xiao Qiang certainly doesn't know.
This sentence means that you can lock the birthday time in
June 4, June 7
September1September 5
Xiao Qiang said: I didn't know at first, but now I know.
June 4th and September 1 will be fine (in Xiao Ming's view).
Xiao Ming said: Oh, I know that, too.
Locked on June 4.