I know the activity is calculate in below formula:-
time = number of two-week periods in which you've posted since your registration
activity = min(time * 14, posts)
However, I registered on 12-Jan, last Tues, why I got 28 activity i/o 14 now?
so when will I promote to Jr. Member? 2-Feb?
January 19 is the date when activity points was awarded to those who reached substantial posts in this forum in which you did, next activity increase will be by February 2 and you'll be Jr member by that time.
Just to clarify that the two week period means the forum's two week period that is same for everybody, which means two week period isn't calculated from the date you registered, but its calculated from when the forum was started.